THE  UNIVERSITY 


OF  ILLINOIS 
LIBRARY 


Digitized  by  the  Internet  Archive 
in  2015 


https://archive.org/details/scientificdialogOOjoyc 


It jti.-il  rr;^/'rsrn/'i//'i'n  rTilir  cnrfh  ^ 


VVBLA  S  I J  i:  I )  J IV   SCO  T  T  .  W  I :  li  s  T  I ;  K  i-  G  I .  A  1  n 


cf'the 

.rV  OF  ILL'' 


SCIENTIFIC  DIALOGUES; 

INTENDED  FOR  THE 

INSTRUCTION  AND  EIVJTERTAINMENT 

OF 

YOUNG  PEOPLE : 

IN  WHICH  THE  FIRST  PRINCIPLES 
OF 

NATURAL  AND  EXPERIMENTAL  PHILOSOPHY 

ARE  FULLY  EXPLAINED. 

BY  THE  REV.  J.  JOYCE. 


NEW  EDITION,  COMPLETE  IN  ONE  VOLUME, 
,   WITH  185  WOOD  CUTS. 


"Conversation,  with  the  habit  of  explaining  the  meanm|  o^^^^^^^^^ 


LONDON : 

PRINTED  FOR  SCOTT,  WEBSTER,  AND  GEARY, 

(SUCCESSORS  TO  MR.  DOVE) 
36,  CHARTERHOUSE  SQUARE. 


PREFACE. 


i      The  Author  feels  himself  extremely  happy  in  the 
S   opportunity  which  this  publication  afFords  him  of 
^   acknowledging  the  obligations  he  is  under  to  the 
^    authors  of  *  Practical  Education/  for  the  pleasure  and 
^    instruction  which  he  has  derived  from  that  valuable 
work.    To  this  he  is  solely  indebted  for  the  idea  of 
writing  on  the  subject  of  Natural  Philosophy  for  the 
^    use  of  children.    How  far  his  plan  corresponds  with 
that  suggested  by  Mr.  Edgeworth,  in  his  chapter  on 
^  Mechanics,  must  be  left  with  a  candid  public  to 
decide. 

The  Author  conceives,  at  least,  he  shall  be  justified 
^  in  asserting,  that  no  introduction  to  natural  and  ex- 

1023555 


iv 


PREFACE. 


perimental  philosophy  has  been  attempted  in  a 
method  so  familiar  and  easy  as  that  which  he  now 
offers  to  the  public  none  which  appears  to  him 
so  properly  adapted  to  the  capacities  of  young  people 
of  ten  or  eleven  years  of  age,  a  period  of  life  which, 
from  the  Author's  own  experience,  he  is  confident  is 
by  no  means  too  early  to  induce  in  children  habits  of 
scientific  reasoning.  In  this  opinion  he  is  sanctioned 
by  the  authority  of  Mr.  Edgeworth.  "Parents," 
says  he,  "are  anxious  that  children  should  be  conver- 
sant with  mechanics,  and  with  what  are  called  the 
mechanical  cowers.  Certainly  no  species  of  know- 
ledge is  better  suited  to  the  taste  and  capacity  of 
youth,  and  yet  it  seldom  forms  a  part  of  early  in- 
struction. Every  body  talks  of  the  lever,  the 
wedge,  and  the  pulley,  but  most  people  perceive 
that  the  notions  which  they  have  of  their  respective 
uses  is  unsatisfactory  and  indistinct,  and  many  en- 
deavour, at  a  late  period  of  life,  to  acquire  a  scientific 
and  exact  knowledge  of  the  effects  that  are  produced 
by  implements  which  are  in  every  body's  hands,  or 
that  are  absolutely  necessary  in  the  daily  occupations 
of  mankind." 

The  Author  trusts  that  the  whole  work  will  be 
found  a  complete  compendium  of  natural  and  ex- 


PREFACE. 


V 


periraental  philosophy,  not  only  adapted  to  the 
understandings  of  young  people,  but  well  calculated 
also  to  convey  that  kind  of  familiar  instruction  which 
is  absolutely  necessary  before  a  person  can  attend 
public  lectures  in  these  branches  of  science  with 
advantage.  "  If,"  says  Mr.  Edgeworth,  speaking 
on  this  subject,  "the  lecturer  does  aot  communicate 
much  of  that  knowledge  which  h3  endeavours  to 
explain,  it  is  not  to  be  attributed  either  to  his  want  of 
skill,  or  to  the  insufficiency  of  his  apparatus,  but  to 
the  novelty  of  the  terms  which  he  is  obliged  to  use. 
Ignorance  of  the  language  in  which  any  science  is 
taught,  is  an  insuperable  bar  to  its  being  suddenly 
acquired ;  besides  a  precise  knowledge  ol  the  meaning 
of  terms,  we  must  have  an  instantaneous  idea  excited 
in  our  minds  whenever  they  are  repeated  ;  and,  as 
this  can  be  acquired  only  by  practice,  it  is  impossible 
that  philosophical  lectures  can  be  of  much  service  to 
those  who  are  not  familiarly  acquainted  with  the 
technical  language  in  which  they  are  deliveied."  * 

*  Mr.  Edgeworth's  chapter  on  Mechanics  sh«uld  be 
recommended  to  the  attention  of  the  reader,  lut  the 
author  feels  unwilling  to  refer  to  part  of  a  wor:,  the 
whole  of  which  deserves  the  careful  perusal  of  al  per- 
sons engaged  in  the  education  of  youth. 

A  2 


PREFACE. 


It  is  presumed  that  an  attentive  perusal  of  these 
Dialogues,  in  which  the  principal  and  most  common 
terms  of  science  are  carefully  explained  and  illus- 
trated, by  a  variety  of  familiar  examples,  will  be  the 
means  of  obviating  this  objection,  with  respect  to 
persons  who  may  be  desirous  of  attending  those  pub- 
lic philosophical  lectures,  to  which  the  inhabitants  of 
the  metropolis  have  almost  constant  access. 


CONTENTS. 


MECHANICS. 


Of  the  Divisibility 


I.  Introduction 
II.  Of  Matter. 
Matter 

III.  Of  the  Attraction  of  Cohesion 

IV.  Of  the  Attraction  of  Cohesion 
V.  Of  the  Attraction  of  Gravitation 

VI.  Of  the  Attraction  of  Gravitation 
VII.  Of  the  Attraction  of  Gravitation 
VIII.  Of  the  Attraction  of  Gravitation 
IX.  Of  the  Centre  of  Gravity  . 

X.  Of  the  Centre  of  Gravity  . 
XI.  Of  the  Laws  of  Motion 
XII.  Of  the  Laws  of  Motion 

XIII.  Of  the  Laws  of  Motion 

XIV.  Of  the  Mechanical  Powers 
XV.  Of  the  Lever 

XVI.  Of  the  Lever  . 

XVII.  Of  the  Wheel  and  Axis 

XVIII.  Of  the  Pulley  . 
XIX.  Of  the  Inclined  Plane 


viii  CONTENTS. 

Conversation  Paffe 

XX.  Of  the  Wedge  65 

XXI.  Of  the  Screw  67 

XXII.  Of  the  Pendulum       ....  71 

ASTRONOMY. 

I.  Of  the  fixed  Stars      ....  74 
II.  Of  the  fixed  Stars     ....  77 

III.  Of  the  fixed  Stars  and  Ecliptic  .  .81 

IV.  Of  the  Ephemeris  .  .  .  .85 
V.  Of  the  Solar  System  .       .       .  .90 

VI.  Of  the  Figure  of  the  Earth        .       .  94 
VII.  Of  the  diurnal  Motion  of  the  Earth     .  98 
VIII.  Of  Day  and  Night    .       .       .  .103 
IX.  Of  the  annual  Motion  of  the  Earth     .  106 

X.  Of  the  Seasons  108 

XI.  Of  the  Seasons  Ill 

XII.  Of  the  Equation  of  Time    .       .  .117 

XIII.  Of  Leap  Year  121 

XIV.  Of  the  Moon  124 

XV.  Of  Eclipses  128 

XVI.  Of  the  Tides  132 

XVII.  Of  the  Harvest  Moon         .       .  .137 

XVIII.  Of  Mercury  140 

XIX.  Of  Venus  142 

XX.  Of  Mars   146 

XXI.  Of  Jupiter  149 

XXII.  Of  Saturn  150 

XXIII.  Of  the  Herschel  Planet     .       .  .154 

XXIV.  Of  Comets  155 

XXV.  Of  the  Sun  157 

XXVI.  Of  the  fixed  Stars       .       .       .       .  ib. 


CONTENTS. 

HYDROSTATICS. 

Conversation 

Page 

T 

Introduction     »       .       •       •  • 

162 

TT 
1J-. 

Of  the  \Veight  and  Pressure  of  Fluids 

loo 

TTT 
111. 

Of  the  Weight  and  Pressure  of  Fluids 

1/1 

TV 
X  V  • 

Of  the  Lateral  Pressure  of  Fluids 

1  /  D 

V  « 

Of  the  Hydrostatic  Paradox  . 

I/O 

V  1. 

\Jl  me  JuLyQlOoLclllC  XJciiuwa            •  • 

183 

V  il. 

Of  the  Pressure  of  Fluids  against  the 

olQcS  Ul   V  cisscla       •           •           •  • 

loo 

VTTT 
V  ill* 

Of  the  Motion  of  Fluids 

190 

1  A. 

Of  tViP  Mntlnn  nf  Fluids 

iy4 

,  X. 

Of  the  Specific  Gravity  of  Bodies 

lyy 

Ai. 

Of  tha  Snppifir*  rrravitv  of  Bodies 

201 

XII. 

Of  the  Methods  of  finding  the  Specific 

Gravity  of  Bodies  .       .       .  • 

2u5 

XIII. 

Of  the  Methods  of  finding  the  Specific 

Gravity  of  Bodies  ...» 

V  • 

Of  the  Methods  of  finding  the  Specific 

Gravity  of  Bodies  .       .       •  « 

213 

XV. 

Of  the  Methods  of  finding  the  Specific 

Gravity  of  Bodies  .       .       .  . 

ZlD 

XVI. 

Of  the  Hydrometer  .... 

XVII. 

Of  the  Hydrometer  and  Svi^imming 

OO/I 
4Z'± 

XVIII. 

Of  the  Syphon  and  Tantalus's  Cup  . 

ZZi 

XIX. 

Of  the  Diver's  Bell    .       .       .  . 

Zol 

XX. 

Of  the  Diver's  Bell   .       .       .  . 

234 

XXI.  Of  Pumps        .       .       .       .  . 

237 

XXII.  Of  the  Forcing-pump— Fire-engine— 


Kope-pump  —  Chain-pump  —  and 
Water-press  .       .  • 

i 


CONTENTS. 


PNEUMATICS. 

Conversation 

I.  Of  the  Nature  of  Air        .       .  .245 
II.  Of  the  Air-pump      .       .       .  .247 

III.  Of  the  Torricellian  Experiment  .       .  252 

IV.  Of  the  Pressure  of  the  Air  .       .       .  255 
V.  Of  the  Pressure  of  the  Air  .       .       .  258 

VI.  Of  the  Weight  of  Air        ...  261 
VII.  Of  the  Elasticity  of  Air      .       .       .  265 
VIII.  Of  the  Compression  of  Air        .       .  270 
IX.  Miscellaneous  Experiments  on  the  Air- 
pump   274 

X.  Of  the  Air-gun  and  Sound        .       .  277 

XI.  Of  Sound  281 

XII.  Of  the  Speaking  Trumpet  .       .  .285 

Jilll.  Of  the  Echo  288 

XIV.  Of  the  Echo  292 

XV.  Of  the  Winds  296 

XVI.  Of  the  Steam-engine  .       .       .  .302 

XVII.  Of  the  Steam-engine  .       .       .  .306 

XVIII.  Of  the  Steam-engine  and  Papin's  Di- 
gester ......  309 

XIX.  Of  the  Barometer     .       .       .  .313 

XX.  Of  the  Barometer,  and  its  Application  to 

the  Measuring  of  Altitudes    .  .317 
XXI.  Of  the  Thermometer  .       .       .  .320 

XXII.  Of  the  Thermometer  .       .       .  .324 

XX II I.  Of  the  Pyrometer  and  Hygrometer  .  327 
X'vIV.  Of  the  Kain-gauge,    and  Rules  for 

judging  of  the  Weather  .       .       .  332 


CONTENTS. 


xi 


OPTICS. 

Conversation  Page 
I.  Light :  the  Smallness  and  Velocity  of 

its  Particles  337 

II.  Kays  of  Light : — Reflection  and  Re- 
fraction       .       .       .       .       .  341 
IIL  Refraction  of  Light   .  .  .344 

IV.  Refraction  and  Reflection  of  Light     .  348 
V.  Different  Kinds  of  Lenses  .       .  .352 
VI.  Parallel  diverging  and  converging  Rays  356 
VII.  Images  of  Objects.  —  Scioptric  Ball, 

&c  360 

VIII.  Nature  and  Advantages  of  Light        e  364 

IX.  Colours  367 

X.  Reflected  Light  and  Plain  Mirrors  .  370 
XI.  Concave  Mirrors  .  .  .  .373 
XII.  Concave  Mirrors. — Experiments.       .  376 

XIII.  Concave  and  Convex  Mirrors     .       .  378 

XIV.  Optical  Deceptions,  Anamorphoses,  &c.  382 
XV.  Different  Parts  of  the  Eye        .       .  385 

XVI.  Manner  of  Vision     .       .       .  .388 

XVII.  Spectacles,  and  their  Uses  .       .       .  392 

XVIII.  Rainbow  396 

XIX.  Refracting  Telescope        .       .  .400 

XX.  Reflecting  Telescopes        .       .       .  404 
XXI.  Microscope       .       .       .       .  .407 
XXII.  Camera   Obscura,   Magic  Lanthorn, 

and  Multiplying  Glass  .       .  .413 

MAGNETISM. 

I.  The  Magnet  418 

II.  Magnetic  Attraction  and  Repulsion     .  420 

III.  Methods  of  making  Magnets      ,       .  423 

IV.  Mariner's  Compass  .       .       ,  427 


xii 


CONTENTS. 


ELECTRICITY. 

Conversaaon  ^  p^^^ 

I.  Early  History  of  Electricity  .  .431 
II.  Electrical  Attraction  and  Repulsion    .  433 

III.  Electrical  Machine   ....  438 

IV.  Electrical  Machine  ....  442 
V.  Electrical  Attraction  and  Repulsion    .  446 

VI.  Electrical  Attraction  and  Repulsion    .  452 
VII.  The  Leyden  Phial     ....  454 
VIII.  Lane's  Electrometer,  and  the  Electrical 

Battery  459 

IX.  Experiments  with  the  Battery     .       .  463 
X.  Miscellaneous  Experiments        .       .  468 
XI.  Electrophorus. — Electrometer. — Thun- 
der-House, &c  472 

XII.  Atmospherical  Electricity  .       .       .  475 

XIII.  Of  Atmospheric  Electricity — of  Falling 

Stars  —  Aurora  Borealis  —  Water- 
Spouts  and  Whirlwinds  —  Earth- 
quakes  479 

XIV.  Medical  Electricity    ....  485 
XV.  Animal  Electricity  —  of  the  Torpedo — 

of  the  Gymnotus  Electricus — of  the 
Silurus  Electricus  .       .       .  .487 
XVI.  General  Summary  of  Electricity,  with 

Experiments        .       .       .  .491 

GALVANISM. 

I.  Of  Galvanism;  its  Origin;  Experiments 

— of  the  Decomposition  of  Water  .  496 
II.  Galvanic  Light  and  Shocks.       .       .  499 

III.  Galvanic  Conductors — Circles — Tables 

— Experiments     ....  503 

IV.  Miscellaneous  Experiments  .  .  509 
Glossary  and  Index  515 


M  E  C  H  A  N  1  C  S. 


CONVERSATION  I. 
INTRODUCTION. 

FATHER  CHARLES — EMBIA, 

Charles.  Father,  you  told  sister  Emma  and  me,  that, 
after  we  had  finished  reading  the  Evenings  at  Home/' 
you  would  explain  to  us  some  of  the  principles  of 
natural  philosophy  :  will  you  begin  this  morning  ? 

Father.  Yes,  I  am  quite  at  leisure  ;  and  I  shall, 
indeed,  at  all  times  take  a  delight  in  communicating  to 
you  the  elements  of  useful  knowledge  ;  and  the  more 
so  in  proportion  to  the  desire  vv^hich  you  have  of  col- 
lecting and  storing  those  facts  that  may  enable  you  to 
understand  the  operations  of  nature,  as  well  as  the 
works  of  ingenious  artists.  These,  I  trust,  will  lead 
you  insensibly  to  admire  the  wisdom  and  goodness  hy 
means  of  which  the  whole  system  of  the  universe  is 
constructed  and  supported. 

Emma.  But  can  philosophy  be  comprehended  by 
children  so  young  as  we  are  1  I  thought  that  it  had 
been  the  business  of  men,  and  of  old  men  too. 

F.  Philosophy  is  a  word  which  in  its  original  sense 
signifies  only  a  love  or  desire  of  wisdom  ;  and  you  will 
not  allow  that  you  and  your  brother  are  too  young  to 
wish  for  knowledge. 

E,  So  far  from  it,  that  the  more  knowledge  I  get 
the  better  I  seem  to  like  it ;  and  the  number  of  new 
ideas  which,  with  a  little  of  your  assistance,  I  have 
obtained  from  the  Evenings  at  Home,''  and  the  great 
.pleasure  which  I  have  received  from  the  perusal  of 
that  work,  will,  I  am  sure,  excite  me  to  read  it  again 
and  again. 

B 


2 


MECHANICS. 


F.  You  will  find  very  little,  in  the  introductory  parts 
of  natural  and  experimental  philosophy,  that  will  re- 
quire more  of  your  attention  than  many  parts  of  that 
work  with  which  you  were  so  delighted. 

C.  But  in  some  books  of  natural  philosophy,  which 
I  have  occasionally  looked  into,  a  number  of  new  and 
uncommon  words  have  perplexed  me  ;  1  have  also 
seen  references  to  figures,  by  means  of  large  letters 
and  small,  the  use  of  which  1  did  not  comprehend. 

F.  It  is  frequently  a  dangerous  practice  for  young 
minds  to  dip  into  subjects  before  they  are  prepared,  by 
some  previous  knowledge,  to  enter  upon  them ;  since 
it  may  create  a  distaste  for  the  most  interesting  topics. 
Thus,  those  books  which  you  now  read  with  so  much 
pleasure  would  not  have  afforded  you  the  smallest  en- 
tertainment a  few  years  ago,  when  you  must  have 
spelt  out  almost  every  word  in  each  page.  The  same 
sort  of  disgust  will  naturally  be  felt  by  persons  who 
should  attempt  to  read  works  of  science  before  the 
leading  terms  are  explained  and  understood.  The 
word  angle  is  continually  recurring  in  subjects  of  this 
sort ;  do  you  know  what  an  angle  is  ? 

E.  I  do  not  think  I  do ;  will  you  explain  what  it 
means  ? 

F.  An  angle  is  made  by  the  opening  of  two  straight* 
lines.  In  this  figure  there  are  two  straight  a 

lines  ah  and  cb  meeting  at  the  point  b,  and    ^  ^-"'^ 

the  opening  made  by  them  is  called  an     y[o-  1. 
angle.  °' 

C.  Whether  that  opening  be  small  or  great,  is  it 
still  called  an  angle  1 

F.  It  is  ;  your  drawing  compasses  may  familiarize 
to  your  mind  the  idea  of  an  angle;  the  lines  in  this 
figure  will  aptly  represent  the  legs  of  the  compasses, 
and  the  point  /;  the  joint  upon  which  they  move  or 
turn.  Now  you  may  open  the  legs  to  any  distance 
you  please,  even  so  far  that  they  shall  form  one  straight 
line  ;  in  tliat  position  only  they  do  not  form  an  angle. 

*  Straight  lines,  in  works  of  science,  are  usually  de- 
nominated 7'ight  lilies. 


INTRODUCTION.  3 
In  every  other  situation  an  angle  is  made  by  the  open- 
ing of  these  legs,  and  the  angle  is  said  to  be  greater 
or^less,  as  that  opening  is  greater  or  less.  An  angle 
is  another  word  for  a  coriier, 

E.  Are  not  some  angles  called  right  angles  ? 

F,  Angles  are  either  right,  acute,  or  obtuse.  When 
the  line  ah  meets  another  line  cd  in  such 

a  manner  as  to  make  the  angles  aha  and 

ahc  equal  to  one  another,  then  those  angles  s  l_A 

are  called  right  angles.    And  the  line  ab 
is  said  to  be  perpendicular  to  cd.   Hence        5*  ; 
to  be  perpendicular  to,  or  to  make  right  angles  with, 
a  line,  means  one  and  the  same  thing. 

C.  Does  it  signify  how  you  call  the  letters  of  an 
angle  1  , 

F.  It  is  usual  to  call  every  angle  by  three  letters, 
and  that  at  the  angular  point  must  be  always  a 
the  middle  letter  of  the  three.    There  are 
cases,  however,  where  an  angle  may  be  deno- 
minated by  a  single  letter  ;  thus  the  angle  abc 
may  be  called  simply  the  angle      for  there  X,^^^ 
is  rio  danger  of  mistake,  because  there  is  but 
a  single  angle  at  the  point  b,  ,n\ 

C,  I  understand  this  ;  for  if,  m  the  second  figure,  i 
were  to  describe  the  angle  by  the  letter  b  only,  you 
would  not  know  whether  I  meant  the  angle  abc  or 
abd, 

F.  That  is  the  precise  reason  why  it  is  necessary 
in  most  descriptions  to  make  use  of  three  letters.  An 
acute  angle  (Fig.  1.)  ahc  is  less  than  a  right  angle  ; 
and  an  obtuse  angle  (Fig.  3.)  abc  is  greater  than  a 
Hght  angle. 

E.  You  see  the  reason  now,  Charles,  why  letters 
are  placed  against  or  by  the  figures,  which  puzzled 
you  before. 

C,  I  do  ;  they  are  intended  to  distmguish  the  sepa- 
rate parts  of  each  in  order  to  render  the  description  of 
them  easier  both  to  the  author  and  the  reader. 

E.  What  is  the  difference,  papa,  between  an  angle 
and  a  triangle  1 


4  MECHANICS. 

F.  An  angle  being  made  by  the  opening  of  two 
lines,  and  as  you  know  that  two  straight  lines  cannot 
enclose  a  space,  so  a  triangle  ahc  is  a  space 
bounded  by  three  straight  lines.   It  takes  ^  X     X  r 
its  name  from  the  property  of  containmg    '"y.^  ^ 
three  angles.    Inhere  are  various  sorts  of  ° 
triandes,  but  it  is  not  necessary  to  enter  upon  these 
particulars,  as  I  do  not  wish  to  burden  your  memories 
with  more  technical  terms  than  we  have  occasion  for. 

C  A  triangle,  then,  is  a  space  or  figure  containing 
three  ancrles,  °and  bounded  by  as  many  straight  Imes. 

F.  Yes,  that  description  will  answer  our  present 
purpose. 


CONVERSATION  II. 

OF  MATTER.  OF  THE  DIVISIBILITY  OF  MATTER. 

F.  Do  you  understand  what  philosophers  mean 
when  they  make  use  of  the  word  matter? 

E.  Are  not  all  things  which  we  see  and  feel  com- 
posed of  matter  1,  .       r  • 

F.  Every  thing  which  is  the  object  of  our  senses  is 
composed  of  matter  differently  modified  or  arranged. 
But  m  a  philosophical  sense  matter  is  defined  to  be  an 
extended,  solid,  inactive,  and  moveable  substance. 

C  If  by  extension  is  meant  length,  breadth,  and 
thickness,  matter,  undoubtedly,  is  an  extended  sub- 
stance. Its  solidity  is  manifest  by  the  resistance  it 
makes  to  the  touch. 

E.  And  tlie  other  properties  nobody  will  deny,  tor 
all  material  objects  are  of  themselves  ^^athout  motion  ; 
and  vet  it  may  be  readily  conceived,  that,  by  applica- 
tion of  a  proper  force,  there  is  no  body  which  cannot 
be  moved.  But  I  remember,  papa,  that  you  told  us 
something  strano-e  about  the  divisibility  of  matter,  which 
vou  said  mii,dit  be  continued  without  end. 
"  F  I  did,  some  time  back,  mention  this  curious  and 
interesting  subject,  and  this  is  a  very  fit  time  tor  me 
to  explain  it. 


DIVISIBILITY  OF  MATTER.  5 

C.  Can  matter  indeed  be  infinitely  divided  ;  for  I 
suppose  that  this  is  what  is  meant  by  a  division  with- 
out end  ?  T  1  •  1 

F.  Difficult  as  this  may  at  first  appear,  yet  I  thmk 
it  very  capable  of  proof.  Can  you  conceive  of  a  par- 
ticle of  matter  so  small  as  not  to  have  an  upper  and 
under  surface  ? 

C.  Certainly  every  portion  of  matter,  however  mi- 
nute, must  have  two  surfaces  at  least,  and  then  I  see 
that  it  follows  of  course  that  it  is  divisible ;  that  is, 
the  upper  and  lower  surfaces  may  be  separated. 

jP.  Your  conclusion  is  just;  and,  though  there 
may  be  particles  of  matter  too  small  for  us  actually 
to  divide,  yet  this  arises  from  the  imperfection  of  our 
instruments ;  they  must  nevertheless,  in  their  nature, 
be  divisible. 

E.  But  you  v^^ere  to  give  us  some  remarkable  m- 
stances  of  the  minute  division  of  matter. 

F,  A  few  years  ago  a  lady  spun  a  single  pound  of 
wool  into  a  thread  168,000  yards  long.^  And  Mr. 
Boyle  mentions  that  two  grains  aM  a  half  of  silk  was 
spun  into  a  thread  300  yards  in  length.  If  a  pound 
of  silver,  which,  you  know,  contains  5,760  grains, 
and  a  single  grain  of  gold,  be  melted  together,  the 
gold  will  be  equally  dififused  through  the  whole  sil- 
ver, insomuch,  that  if  one  grain  of  the  mass  be  dis- 
solved in  a  liquid  called  aqua  forth,  the  gold  will  fall 
to  the  bottom.  By  this  experiment,  it  is  evident  that 
a  grain  may  be  divided  into  5,761  visible  parts,  for 
only  the  5,761st  part  of  the  gold  is  contained  in  a 
single  grain  of  the  mass. 

The  goldbeaters,  whom  you  have  seen  at  work  in 
the  shops  in  Long-acre,  can  spread  a  grain  of  gold 
into  a  leaf  containing  50  square  inches,  and  this  leaf 
may  be  readily  divided  into  500,000  parts,  each  of 
which  is  visible  to  the  naked  eye  :  and  by  the  help  of 
a  microscope,  which  magnifies  the  area  or  surface  of 
a  body  100  times,  the  100th  part  of  each  of  these  be- 
comes visible ;  that  is,  the  50  millionth  part  of  a 
grain  of  gold  will  be  visible,  or  a  single  grain  of  that 


6  MECHANICS. 

metal  may  be  divided  into  50  millions  of  visible  parts. 
But  the  gold  which  covers  the  silver  wire  used  m 
making  what  is  called  gold  lace,  is  spread  over  a  much 
larger  surface,  yet  it  preserves,  even  if  exammed  by  a 
microscope,  an  uniform  appearance.  It  has  been 
calculated  that  one  grain  of  gold,  under  these  circum- 
stances, would  cover  a  surface  of  nearly  thirty  square 
yards. 

The  natural  divisions  of  matter  are  still  more  sur- 
prising. In  odoriferous  bodies,  such  as  camphor, 
musk,  and  assafoetida,  a  wonderful  subtilty  of  parts  is 
perceived  ;  for,  though  they  are  perpetually  filling  a 
considerable  space  with  odoriferous  particles,  yet  these 
bodies  lose  but  a  very  small  part  of  their  weight  in  a 
great  length  of  time.  . 

Again,  it  is  said  by  those  who  have  examined  the 
subject  with  the  best  glasses,  and  whose  accuracy  may 
be  relied  on,  that  there  are  more  animals  in  the  milt 
of  a  single  cod-fish,  than  there  are  men  on  the  whole 
earth,  and  that  a  single  grain  of  sand  is  larger  than 
four  millions  of  these  animals.  Now  if  it  be  admitted 
that  these  little  animals  are  possessed  of  organised 
parts,  such  as  a  heart,  stomach,  muscles,  veins,  arte- 
ries, &c.  and  that  they  are  possessed  of  a  complete 
system  of  circulating  fluids,  similar  to  what  is  tound 
in  larger  animals,  we  seem  to  approach  to  an  idea  of 
the  infinite  divisibility  of  matter.  It  has  indeed  been 
calculated,  that  a  particle  of  the  blood  of  one  ot  these 
animalculse  is  as  much  smaller  than  a  globe  one-tenth 
of  an  inch  in  diameter,  as  that  globe  is  smaller  than 
the  whole  earth.  Nevertheless,  if  these  particles  be 
compared  with  the  particles  of  light,  it  is  probable 
that  they  would  be  found  to  exceed  them  m  bulk  as 
much  as  mountains  do  single  grains  of  sand. 

I  mioht  enumerate  many  other  instances  ot  the 
same  kind,  but  these,  I  doubt  not,  will  be  sufficient  to 
convince  you  into  what  very  minute  parts  matter  is 
capable  of  being  divided.  ,  ,  i 

Captain  Scoresby,  in  his  Account  ot  the  Greenland 
Seas,  states,  that,  in  July  1818,  his  vessel  sailed  for 


ATTRACTION  OF  COHESION.  7 

several  leagues  in  water  of  a  very  uncommon  appear- 
ance The  surface  was  variegated  by  large  patches 
of  a  yellowish-green  colour.  It  was  found  to  be  pro- 
duced by  animalculse,  and  microscopes  were  applied 
to  their  examination.  In  a  single  drop  of  the  water, 
examined  by  a  power  of  28,224  (magnified  superfi- 
cies), there  were  50  in  number,  on  an  average,  in 
each  square  of  the  micrometer  glass  of  l-340th  ot  an 
inch  in  diameter;  and,  as  the  drop  occupied  a  circle 
on  a  plate  of  glass  containing  529  of  these  squares, 
there  must  have  been  in  this  single  drop  of  water, 
taken  at  random  out  of  the  sea,  and  in  a  place  not  the 
most  discoloured,  about  26,450  animalculae.  How 
inconceivably  minute  must  the  vessels,  organs,  and 
fluids,  of  these  animals  be  !  A  whale  requires  a  sea 
to  sport  in  :  a  hundred  and  fifty  millions  oj  these  would 
have  ample  scope  for  their  evolutions  in  a  tumbler  oJ 
water! 

CONVERSATION  III. 

OF  THE   ATTRACTION   OF  COHESION. 

F  Well,  my  dear  children,  have  you  reflected 
upon  what  we  last  conversed  about?  Do  you  com- 
prehend the  several  instances  which  I  enumerated  as 
examples  of  the  minute  division  of  matter] 

E.  Indeed,  the  examples  which  you  gave  us  very 
much  excited  my  wonder  and  admiration,  and  yet, 
from  the  thinness  of  some  leaf  gold  which  I  once 
had,  I  can  readily  credit  all  you  have  said  on  that 
part  of  the  subject.  But  I  know  not  how  to  conceive 
of  such  small  animals  as  you  described  ;  and  I  am  still 
more  at  a  loss  how  to  imagine  that  ariimals  so  minute 
^  should  possess  all  the  properties  of  the  larger  ones, 
such  as  a  heart,  veins,  blood,  &c.       ,     ,     ,  ^  . 

F  I  can,  the  next  bright  morning,  by  the  help  ot 
my  solar  microscope,  shew  you  very  distinctly,  the 
circulation  of  the  blood  in  a  flea,  which  you  may  get 
from  your  little  dog;  and  with  better  glasses  than 
those  of  which  1  am  possessed,  the  same  appearance 


g  MECHANICS, 
might  be  seen  in  animals  still  smaller  than  the  flea, 
perhaps  even  in  those  which  are  themselves  invisible 
to  the  naked  eye.  But  we  shall  converse  more  at 
large  on  this  matter,  when  we  come  to  consider  the 
subject  of  optics,  and  the  construction  and  uses  of  the 
solar  microscope.  At  present  we  will  turn  our 
thoughts  to  that  principle  in  nature,  which  philoso- 
phers have  agreed  to  call  gravity,  or  attraction. 

C.  If  there  be  no  more  difficulties  in  phdoso- 
phy  than  we  met  with  in  our  last  lecture,  1  do  not 
fear  but  that  we  shall,  in  general,  be  able  to  under- 
stand it.    Are  there  not  several  kinds  of  attraction  1 

F.  Yes,  there  are  ;  two  of  which  it  will  be  sufficient 
for  our  present  purpose  to  describe ;  the  one  is  the 
attraction  of  cohesion;  the  other,  that  of  gravitation. 
The  attraction  of  cohesion  is  that  power  which  keeps 
the  parts  of  bodies  together  when  they  touch,  and 
prevents  them  from  separating,  or  which  inclines  the 
parts  of  bodies  to  unite,  when  they  are  placed  suffi- 
ciently near  to  each  other. 

C.  Is  it  then  by  the  attraction  of  cohesion  that  the 
parts  of  this  table,  or  of  the  penknife,  are  kept  to- 
gether ? 

F.  The  instances  which  you  have  selected  are  ac- 
curate, but  you  might  have  said  the  same  of  every 
other  solid  substance  in  the  room  ;  and  it  is  in  propor- 
tion to  the  different  degrees  of  attraction  with  which 
different  substances  are  affected,  that  some  bodies  are 
hard,  others  soft,  tough,  &c.  A  philosopher  in  Hol- 
land, almost  a  century  ago,  took  great  pains  in  ascer- 
taining the  different  degrees  of  cohesion  which  be- 
longed to  various  kinds  of  wood,  metals,  and  many 
other  substances.  A  short  account  of  the  experiments 
made  by  M.  Musschenbroek,  you  will^  hereafter  find 
in  your  own  language,  in  Dr.  Enfield's  Institutes  of 
Natural  Philosophy. 

C.  You  once  shewed  me  that  two  leaden  bullets 
having  a  little  scraped  from  the  surfaces,  would  stick 
together  with  great  force  ;  you  called  that,  1  believe, 
the  attraction  of  cohesion  ? 


ATTRACTION  OF  COHESION.  0 
F.  I  did  :  some  philosophers,  who  have  made  this 
experiment  with  great  attention  and  accuracy,  assert, 
that  if  the  flat  surfaces,  which  are  presented  to  one 
another,  be  but  a  quarter  of  an  inch  in  diameter, 
scraped  very  smooth,  and  forcibly  pressed  together 
with  a  twist,  a  weight  of  a  hundred  pounds  is  tre- 
quently  required  to  separate  them. 

As  it  is  by  this  kind  of  attraction  that  the  parts  ot 
solid  bodies  are  kept  together,  so,  when  any  substance 
is'separated  or  broken,  it  is  only  the  attraction  of  co- 
hesion that  is  overcome  in  that  particular  part. 

E.  Then,  when  I  had  the  misfortune  this  morning 
at  breakfast  to  let  my  saucer  slip  from  my  hands,  by 
which  it  was  broken  into  several  pieces,  was  it  only 
the  attrabtion  of  cohesion  that  was  overcome  by  the 
parts  of  the  saucer  being  separated  by  its  fall  on  the 
ground  ] 

F.  Just  so  ;  for  whether  you  unluckily  break  the 
china,  or  cut  a  stick  with  your  knife,  or  melt  lead  over 
the  fire,  as  your  brother  sometimes  does,  in  order  to 
make  plummets  ;  these  and  a  thousand  other  instances 
which  are  continually  occurring,  are  but  examples  in 
which  the  cohesion  is  overcome  by  the  fall,  the  knife, 
or  the  fire. 

E.  The  broken  saucer  being  highly  valued  by 
mamma,  she  has  taken  the  pains  to  join  it  again  with 
white  lead  ■  was  this  performed  by  means  of  the  at- 
traction of  cohesion  ? 

F.  It  was,  my  dear  ;  and  hence  you  will  easily 
learn  that  many  operations  in  cookery  are  in  fact  no- 
thing more  than  different  methods  of  causing  this 
attraction  to  take  place.  Thus  flour,  by  itself,  has 
little  or  nothing  of  this  principle,  but  when  mixed  with 
milk,  or  other  liquids,  to  a  proper  consistency,  the 
parts  cohere  strongly,  and  this  cohesion  in  many  in- 
stances becomes  still  stronger  by  means  of  the  heat 
applied  to  it  in  boiling  or  baking. 

C.  You  put  me  in  mind  of  the  fable  of  the  man 
blowing  hot  and  cold  ;  for,  in  the  instance  of  the  lead, 
lire  overcomes  the  attraction  of  cohesion;  and  the 
B2 


10  MECHANICS. 

same  power,  heat,  v/hen  applied  to  puddings,  bread, 
&c.  causes  their  parts  to  cohere  more  powerfully. 
How  are  we  to  understand  this  ? 

F.  I  will  endeavour  to  remove  your  difficulty. 
Heat  expands  all  bodies  without  exception,  as  you 
shall  see  before  we  have  finished  our  lectures.  Now 
the  fire  applied  to  metals,  in  order  to  melt  them,  causes 
such  an  expansion,  that  the  particles  are  thrown  out 
of  the  sphere,  or  reach,  of  each  other's  attraction  ; 
whereas  the  heat  communicated  in  the  operation  of 
cookery,  is  sufficient  to  expand  the  particles  of  flour, 
but  is  not  enough  to  overcome  the  attraction  of  co- 
hesion. Besides,  your  mamma  will  tell  you,  that  the 
heat  of  boiling  would  frequently  disunite  the  parts  of 
which  her  puddings  are  composed,  if  she  did  not  take 
the  precaution  of  enclosing  them  in  a  cloth,  leaving 
them  just  room  enough  to  expand  without  the  liberty 
of  breaking  to  pieces  ;  and  the  moment  they  are  taken 
from  the  water,  they  lose  their  superabundant  heat, 
and  become  solid. 

£.  When  Ann  the  cook  makes  broth  for  little  bro- 
ther, it  is  the  heat  then  which  overcomes  the  attrac- 
tion which  the  particles  of  meat  have  for  each  other, 
for  I  have  seen  her  pour  off  the  broth,  and  the  meat 
is  all  in  rags.  But  will  not  the  heat  overcome  the 
attraction  which  the  parts  of  the  bones  have  for  each 
other  ? 

jP,  The  heat  of  boiling  water  will  never  effect  this, 
but  a  machine  was  invented  several  years  ago,  by  Mr. 
Papin,  for  that  purpose.  It  is  called  Papin's  Digester, 
and  is  used  in  taverns,  and  in  many  large  families, 
for  the  purpose  of  dissolving  bones  as  completely  as 
a  lesser  degree  of  heat  will  liquefy  jelly.  On  some 
future  day  I  will  shew  you  an^engraving  of  this  ma- 
chine, and  explain  its  different  parts,  which  are  ex- 
tremely simple.* 


*  See  Pneumatics,  Conversation  XVIII. 


ATTRACTION  OF  COHESION.  11 


CONVERSATION  IV. 

OF  THE  ATTRACTION   OF  COHESION. 

F.  I  will  now  mention  some  other  instances  of  this 
great  law  of  nature.  If  two  polished  plates  of  marble, 
or  brass,  be  put  together,  with  a  little  oil  between 
them  to  fill  up  the  pores  in  their  surfaces,  they  will 
cohere  so  powerfully  as  to  require  a  very  considerable 
force  to  separate  them. — Two  globules  of  quicksilver, 
placed  very  near  to  each  other,  will  run  together  and 
form  one  large  drop. — Drops  of  water  will  do  the 
same. — Two  circular  pieces  of  cork  placed  upon 
water  at  about  an  inch  distant  will  run  together. — 
Balance  a  piece  of  smooth,  board  on  the  end  of  a  scale 
beam  ;  then  let  it  lie  flat  on  water,  and  five  or  six 
times  its  own  weight  will  be  required  to  separate  it 
from  the  water.  If  a  small  globule  of  quicksilver  be 
laid  on  clean  paper,  and  a  piece  of  glass  be  brought 
into  contact  with  it,  the  mercury  will  adhere  to  it, 
and  be  drawn  away  from  the  paper.  But  bring  a 
larger  globule  into  contact  with  the  smaller  one,  and 
it  will  forsake  the  glass,  and  unite  with  the  other 
quicksilver. 

C.  Is  it  not  by  means  of  the  attraction  of  cohe- 
sion, that  the  little  tea  which  is  generally  left  at  the 
bottom  of  the  cup  instantly  ascends  in  the  sugar  when 
thrown  into  it  1 

F,  The  ascent  of  water  or  other  liquids  in  sugar, 
sponge,  and  all  porous  bodies,  is  a  species  of  this  at- 
traction, and  is  called  capillary  *  attraction :  it  is  thus 
denominated  from  the  property  which  tubes  of  a  very 
small  bore,  scarcely  larger  than  to  admit  a  hair,  have 
of  causing  water  to  stand  above  its  level. 

C.  Is  this  property  visible  in  no  other  tubes  than 
those  the  bores  of  which  are  so  exceedingly  fine  ? 

F,  Yes,  it  is  very  apparent  in  tubes  whose  diame- 
ters are  one-tenth  of  an  inch  or  more  in  length,  but 
the  smaller  the  bore,  the  higher  the  fluid  rises  ;  for  it 

*  From  capillus,  the  Latin  word  for  hair. 


12  MECHANICS. 

ascends,  in  all  instances,  till  the  weight  of  the  column 
of  water  in  the  tube  balances,  or  is  equal  to,  the  at- 
traction of  the  tube.  By  immersing  tubes  of  different 
bores  in  a  vessel  of  coloured  water,  you  wil  see  t.iat 
the  water  rises  as  much  higher  in  the  smaller  tube, 
than  in  the  larger,  as  its  bore  is  less  than  that  of  tlie 
larger.  The  water  will  rise  a  quarter  of  an  inch,  and 
there  remain  suspended  in  a  tube,  whose  bore  is  about 
one-eip-hth  of  an  inch  in  diameter. 

This  kind  of  attraction  is  well  illustrated,  by  taking 

two  pieces  of  glass,  joined  together  at      jr-  — 

the  side  be,  and  kept  a  little  open  at  k  J^^ 
the  opposite  side  ad,  by  a  small  piece  ^^^^^^ 
of  cork  e.  In  this  position  immerse  f^^m^ 
them  in  a  dish  of  coloured  water  Jg,  j  -j/^, 

and  you  will  observe  that  the  at  iv"5 

traction  of  the  glass  at  and  near  be  n-  • 

will  cause  the  fluid  to  ascend  to  />,  whereas  about  the 
parts  d,  it  scarcely  rises  above  the  level  of  the  water 
in  the  vessel.  ,  ,  , 

C  I  see  that  a  curve  is  formed  by  tlie  water. 
F  There  is,  and  to  this  curve  there  are  many  cu- 
rious properties  belonging,  as  you  will  hereafter  be 
able  to  investigate  for  yourself. 

E  Is  it  not  upon  the  principle  of  the  attrac- 
tion of  cohesion,  that  carpenters  glue  their  work  to- 

p^ether  1  .  ,     ,  ^  i 

°  F  It  is  upon  this  principle  that  carpenters  and 
cabinet-makers  make  use  of  glue;  that  braziers  tm- 
men,  plumbers,  &c.  solder  their  metals;  and  that 
smiths  unite  different  bars  of  iron  by  means  of  heat. 
These  and  a  thousand  other  operations  of  which  we 
are  ''continually  the  witnesses,  depend  on  the  same 
principle  as  that  which  induced  your  mamma  to  use 
ihe  white  lead  m  mending  her  saucer.  And  you  ought 
to  be  told,  that  though  white  lead  is  frequently  used  as 
u  cement  for  broken  china,  glass,  and  earthenware, 
vet  if  the  vessels  are  to  be  brought  agam  into  use,  it  is 
not'a  proper  cement,  being  an  active  poison  besides^ 
one  much  stronger  has  been  discovered,  1  believe,  by 


ATTRACTION  OF  COHESION,  13 
a  very  able  and  ingenious  philosopher,  the  late  Dr. 
Ingenhouz ;  at  least  I  had  it  from  him  several  years 
ago  ;  it  consists  simply  of  a  mixture  of,  quick-lime  and 
Glocester  cheese,  rendered  soft  by  warm  water,  and 
worked  up  to  a  proper  consistency. 

E.  What!  do  such  great  philosophers,  as  I  have 
heard  you  say  Dr.  Ingenhouz  was,  attend  to  such 
trifling  things  as  these  1 

F.  He  was  a  man  deeply  skilled  in  many  branches 
of  science  ;  and  I  hope  that  you  and  your  brother  will 
one  day  make  yourselves  acquainted  with  many  of 
his  important  discoveries.  But  no  real  philosopher 
will  consider  it  beneath  his  attention  to  add  to  the  con- 
veniences of  life. 

C.  This  attraction  of  cohesion  seems  to  pervade  the 
whole  of  nature. 

F.  It  does,  but  you  will  not  forget  that  it  acts  only 
at  very  small  distances.  Some  bodies  indeed  appear 
to  possess  a  power  the  reverse  of  the  attraction  of  co- 
hesion. 

F,  What  is  that,  papa  ? 

F.  It  is  called  repulsion.  Thus  water  repels  most 
bodies  till  they  are  wet.  A  small  needle  carefully 
placed  on  water  will  swim  :  flies  walk  upon  it  without 
wetting  their  feet :  the  drops  of  dew  which  appear  in 
a  morning  on  plants,  particularly  on  cabbage  plants, 
assume  a  globular  form,  from  the  mutual  attraction 
between  the  particles  of  water  ;  and  upon  examination 
it  will  be  found  that  the  drops  do  not  touch  the  leaves, 
for  they  will  roll  off  in  compact  bodies,  which  could 
not  be  the  case  if  there  subsisted  any  degree  of  at- 
traction between  the  water  and  the  leaf. 

If  a  small  thin  piece  of  iron  be  laid  upon  quicksd- 
ver,  the  repulsion  between  the  different  metals  will 
cause  the  surface  of  the  quicksilver  near  the  iron  to 
be  depressed.  n  •  i  •  i 

The  repelling  force  of  the  particles  of  a  fluid  is  but 
small ;  therefore,  if  a  fluid  be  divided  it  easdy  unites 
again.  But  if  a  glass  or  any  hard  substance  be 
broken,  the  parts  cannot  be  made  to  cohere  without 


14 


MECHANICS. 


being  first  moistened^  because  the  repulsion  is  too 
great  to  admit  of  a  re-union. 

The  repelling  force  between  water  and  oil  is  like- 
wise so  great,  that  it  is  almost  impossible  to  mix  them 
in  such  a  manner  that  they  shall  not  separate  again. 

If  a  ball  of  light  wood  be  dipped  in  oil,  and  then 
put  into  water,  the  water  will  recede  so  as  to  form  a 
small  channel  around  the  ball. 

C.  Why  do  cane,  steel,  and  many  other  things, 
bear  to  be  bent  without  breaking,  and,  when  set  at 
liberty  again,  recover  their  original  form  1 

F.  That  a  piece  of  thin  steel,  or  cane,  recovers  its 
usual  form  after  being  bent,  is  owing  to  a  certain 
power,  called  elasticity,  which  may,  perhaps,  arise 
from  tlie  particles  of  those  bodies,  though  disturbed, 
not  being  drawn  out  of  each  other's  attraction  ;  there- 
fore, as  soon  as  the  force  upon  them  ceases  to  act, 
they  restore  themselves  to  their  former  position. — But 
our  half  hour  is  expired ;  I  must  leave  you. 

CONVERSATION  V. 

OF  THE  ATTRACTION  OF  GRAVITATION. 

F.  We  will  now  proceed  to  discuss  another  very 
important  general  principle  in  nature ;  the  attrac- 
tion of  gravitation,  or,  as  it  is  frequently  termed, 
gravity,  which  is  that  power  by  which  distant  bodies 
tend  towards  each  other.  Of  this  we  have  perpetual 
instances  in  the  falling  of  bodies  to  the  earth. 

C.  Am  I,  then,  to  understand  that  whether  this 
marble  falls  from  my  hand,  or  a  loose  brick  from 
the  top  of  the  house,  or  an  apple  from  the  tree  in  the 
orchard,  that  all  these  happen  by  the  attraction  of 
gravity  1 

F.  It  is  by  the  power  which  is  commonly  expressed 
under  the  term  gravity,  that  all  bodies  whatever  have 
a  tendency  to  the  earth  ;  and,  unless  supported,  will 
fall  in  lines  nearly  perpendicular  to  its  surface. 

E.   But  are  not  smoke,  steam,  and  other  light 


ATTRACTION  OF  GRAVITATION.  15 
bodies,  which  we  see  ascend,  exceptions  to  the  gene. 

ral  rule  ?  ,        ,  •         r  i 

F,  It  appears  so  at  first  sight,  and  it  was  formerly 
received  as  a  general  opinion,  that  smoke,  steam,  &c. 
possessed  no  weight:  the  discovery  of  the  air-pump 
has  shewn  the  fallacy  of  this  notion,  for  m  an  ex- 
hausted receiver,  that  is,  in  a  glass  jar  from  which 
the  air  is  taken  away  by  means  of  the  air-pump, 
smoke  and  steam  descend  by  their  own  weight  as  com- 
pletely as  apiece  of  lead.  When  we  come  to  con- 
verse  on  the  subjects  of  pneumatics  and  hydrostatics 
you  will  understand  that  the  reason  why  smoke  and 
other  bodies  ascend  is  simply  because  they  are  lighter 
than  the  atmosphere  which  surrounds  them,  and  the 
moment  they  reach  that  part  of  it  which  has  the  same 
gravity  with  themselves  they  cease  to  rise. 

C.  Is  it,  then,  by  this  power  that  all  terrestrial  bo- 
dies remain  firm  on  the  earth  1 

F  By  gravity,  bodies  on  all  parts  oi  the  earth 
(which  you  kaow  is  of  a  globular  form)  are  kept  on 
its  surface,  because  they  all,  wherever  situated,  tend 
to  the  centre ;  and,  since  all  have  a  tendency  to  the 
centre,  the  inhabitants  of  New  Zealand,  although 
nearly  opposite  to  our  feet,  stand  as  firm  as  we  do  in 
Great  Britain. 

C.  This  is  difficult  to  comprehend ;  nevertheless, 
if  bodies  on  all  parts  of  the  surface  of  the  earth  have 
a  tendency  to  the  centre,  there  seems  no  reason  why 
bodies  should  not  stand  as  firm  on  one  part  as  well 
as  another.  Does  this  power  of  gravity  act  alike  on 
all  bodies  ?  .       .  •  n 

F,  It  does,  without  any  regard  to  their  figure  or 
size  ;  for  attraction  or  gravity  acts  upon  bodies  in  pro- 
portion to  the  quantity  of  matter  which  they  contain 
that  is,  four  times  a  greater  force  of  gravity  is  exerted 
upon  a  weight  of  four  pounds  than  upon  one  of  a 
single  pound.  The  consequence  of  this  prmciple  is 
that  all  bodies  at  equal  distances  from  the  earth  fall 
with  equal  velocity. 

E.  What  do  you  mean,  papa,  by  velocity  r 


16  MECHANICS. 

F,  I  will  explain  it  by  an  example  or  two  :  if  you 
and  Charles  set  out  together,  and  you  walk  a  mile  in 
half  an  hour,  but  he  walk  and  run  two  miles  in  the 
same  time,  how  much  swifter  will  he  go  than  you  ? 

E.  Twice  as  swift. 

F.  He  does,  because,  in  the  same  time,  he  passes 
over  twice  as  much  space  ;  therefore,  we  say  his  velo- 
city is  twice  as  great  as  your's.  Suppose  a  ball,  fired 
from  a  cannon,  pass  through  800  feet  in  a  second  of 
time,  and  in  the  same  time  your  brother's  arrow  pass 
through  100  feet  only,  how  much  swifter  does  the 
cannon-ball  fly  than  the  arrow  ? 

E.  Eight  times  swifter. 

F.  Then  it  has  eight  times  the  velocity  of  the  ar- 
row ;  and  hence  you  understand  that  swiftness  and 
velocity  are  synonymous  terms  ;  and  that  the  velocity 
of  a  body  is  measured  by  the  space  it  passes  over  in  a 
given  time,  as  a  second,  a  minute,  an  hour,  &c. 

E.  If  I  let  a  piece  of  metal,  as  a  penny-piece,  and 
a  feather,  fall  from  my  hand  at  the  same  time,  the 
penny  will  reach  the  ground  much  sooner  than  the 
feather.  Now  how  do  you  account  for  this  if  all  bo- 
dies are  equally  affected  by  gravitation,  and  descend 
with  equal  velocities,  when  at  the  same  distance  from 
the  earth  ? 

F.  Though  the  penny  and  feather  will  not,  in  the 
open  air,  fall  with  equal  velocity,  yet  if  the  air  be 
taken  away,  which  is  easily  done,  by  a  little  appara- 
tus connected  with  the  air-pump,  they  will  descend  m 
the  same  time.  Therefore  the  true  reason  why  light 
and  heavy  bodies  do  not  fall  with  equal  velocities,  is, 
that  the  Jonner,  in  proportion  to  its  weight,  meets  with 
a  much  greater  resistance  from  the  air  than  the  latter. 

C.  It  is  then,  I  imagine,  from  the  same  cause  that, 
if  I  drop  the  penny  and  a  piece  of  light  v.ood  into  a 
vessel  of  water,  the  penny  shall  reach  the  bottom,  but 
the  wood,  after  descending  a  small  way,  rises  to  the 
surface. 

F.  In  this  case,  the  resi!>ting  medium  is  water  in- 
stead of  air,  and  the  copper,  being  about  nine  times 


ATTRACTION  OF  GRAVITATION.  17 
heavier  than  its  bulk  of  water,  falls  to  the  bottom 
without  apparent  resistance.  But  the  wood,  bemg 
much  liohter  than  water,  cannot  sink  in  it;  therelore, 
thouP-h  by  its  jnomentum''  it  sinks  a  small  distance,  yet, 
as  soon  as  that  is  overcome  by  the  resisting  medium, 
it  rises  to  the  surface,  being  the  lighter  substance. 


CONVERSATION  VI. 

OF  THE  ATTRACTION  OF  GRAVITATION. 

E.  The  term  momentum,  which  you  made  use  of 
yesterday,  is  another  word  which  I  do  not  understand, 

F.  If  you  have  understood  what  I  have  said  re- 
specting the  velocity  of  moving  bodies,  you  will  easily 
comprehend  what  is  meant  by  the  word  momentum  . 

The  momentum,  or  moving  force,  of  a  body,  is  its 
weight  multiplied  into  its  velocity.    You  may,  for 
instance,  place  this  pound  weight  upon  a  chma-plate 
without  any  danger  of  breaking,  but,  if  you  let  it  lall 
from  the  height  of  only  a  few  inches,  it  will  dash  the 
china  to  pieces.    In  the  first  case,  the  plate  has  only 
the  pound  weight  to  sustain  ;  in  the  other,  the  weiglit 
must  be  multiplied  into  the  velocity,  or,  to  speak  in  a 
popular  manner,  into  the  distance  of  the  height  from 
which  it  fell.  ^ 
If  a  ball  a  lean  against  the  obsta-  ^.^^,,3^ 
cle  h,  it  will  not  be  able  to  overturn  e  JlJJ^  g 
it,  but  if  it  be  taken  up  to  c,  and  suf-  j^- 
fered  to  roll  down  the  inclined  plane 
de  against  b,  it  will  certainly  overthrow  it;  in  the 
former  case,  b  would  only  have  to  resist  the  weight  of 
the  ball  a,  in  the  latter  it  has  to  resist  the  weight  mul- 
tiplied into  its  motion,  or  velocity. 

C.  Then  the  momentum  of  a  small  body,  whose 
velocity  is  very  great,  may  be  equal  to  that  of  a  very 
large  body  with  a  slow  velocity. 


*  The  explanation  of  this  term  wilt  be  found  in  the 
next  Conversation. 


18  MECHANICS. 

F.  It  may,  and  hence  you  see  the  reason  why  im- 
mense battering-rams,  used  by  the  ancients  in  the  art 
of  war,  have  given  place  to  cannon  balls  of  but  a  few 
pounds  weight. 

C.  I  do,  for  what  is  wanting  in  weight,  is  made  up  ^ 
by  velocity. 

F.  Can  you  tell  me  what  velocity  a  cannon  ball  of 
28  pounds  must  have,  to  effect  the  same  purposes,  as 
would  be  produced  by  a  battering  ram  of  15,000 
pounds  weight,  and  which,  by  manual  strength,  could 
be  moved  at  the  rate  of  only  two  feet  in  a  second  of 
time  ? 

C.  I  think  I  can  :— the  momentum  of  the  battermg 
ram  must  be  estimated  by  its  weight,  multiplied  into 
the  space  passed  over  in  a  second,  which  is  15,000 
multiplied  by  two  feet,  equal  to  30,QP0  ;  now  if  this 
momentum,  which  must  also  be  that  of  the  cannon 
ball,  be  divided  by  the  weight  of  the  ball,  it  will  give 
the  velocity  required  ;  and  30,000  divided  by  28,  will 
give  for  the  quotient  1072  nearly,  which  is  the  num- 
ber of  feet  which  the  cannon  ball  must  pass  over  in  a 
second,  in  order  that  the  momenta  of  the  battering 
ram  and  the  ball  may  be  equal,  or,  in  other  words, 
that  they  may  have  the  same  effect  in  beating  down 
an  enemy's  wall. 

E,  I  now  fully  comprehend  what  the  momentum 
of  a  body  is,  for  if  I  let  a  common  trap-ball  accident- 
ally fall  from  my  hand  upon  my  foot,  it  occasions 
more  pain  than  the  mere  pressure  of  a  weight  several 
times  heavier  than  the  ball. 

F.  If  you  let  a  pound,  or  a  hundred  pounds,  fall  on 
the  floor,  only  from  the  height  of  an  inch  and  a  quar- 
ter, it  will  strike  the  floor  with  a  momentum  equal  to 
double  its  weight :  and  if  you  let  it  fall  from  four 
times  that  height,  or  five  inches,  it  will  have  double 
that  effect ;— and  if  it  fall  nine  tmies  that  height,  or 
eleven  inches  and  a  quarter,  it  will  have  treble  the 
effect ; — and  by  falling  sixteen  times  the  height,  or 
twenty  inches,  it  will  have  four  times  the  effect,  and 
so  on.    Hence  it  is  plain,  that  if  you  let  the  ball  diop 


ATTRACTION  OF  GRAVITATION.  19 
from  your  hand  at  the  height  of  twenty  inches  only,  it 
will  have  eight  times  more  effect  in  causing  pain  than 
the  mere  pressure  of  the  ball  itself. 

C.  If  the  attraction  of  gravitation  be  a  power  by 
which  bodies  in  general  tend  towards  each  other,  why 
do  all  bodies  tend  to  the  earth  as  a  centre  ? 

F.  I  have  already  told  you  that  by  the  great  law  of 
gravitation,  the  attraction  of  all  bodies  is  in  proportion 
to  the  quantity  of  matter  which  they  contain.  ^  Now 
the  earth,  being  so  immensely  large  in  comparison  of 
ail  other  substances  in  its  vicinity,  destroys  the  effect 
of  this  attraction  between  smaller  bodies,  by  bring- 
ing them  all  to  itself. — If  two  balls  are  let  fall  from  a 
high  tower  at  a  small  distance  apart,  though  they 
have  an  attraction  for  one  another,  yet  it  will  be  as 
nothing  when  compared  with  the  attraction  by  which 
they  are  both  impelled  to  the  earth,  and  consequently 
the  tendency  which  they  mutually  have  of  approaching 
one  another  will  not  be  perceived  in  the  fall.  If, 
however,  any  two  bodies  were  placed  in  free  space,  and 
out  of  the  sphere  of  the  earth's  attraction,  they  would 
in  that  case  assuredly  fall  toward  each  other,  and 
that  with  increased  velocity  as  they  came  nearer.  If 
the  bodies  were  equal,  they  would  meet  in  the  middle 
point  between  the  two;  but  if  they  were  unequal,  they 
would  then  meet  as  much  nearer  the  larger  one,  as 
that  contained  a  greater  quantity  of  matter  than  the 
other. 

C.  According  to  this,  the  earth  ought  to  move  to- 
wards falling  bodies,  as  well  as  they  move  to  it. 

F,  It  ought,  and,  in  just  theory,  it  does:  but  when 
you  calculate  how  many  million  of  times  larger  the 
earth  is  than  any  thing  belonging  to  it ;  and  if  you 
reckon  the  small  distances  from  which  bodies  can  fall, 
you  will  then  know  that  the  point  where  the  falling 
bodies  and  earth  will  meet,  is  removed  only  to  an  in- 
definitely small  distance  from  its  surface  ;  a  distance 
much  too  small  to  be  conceived  by  the  human  imagi- 
nation. 

We  will  resume  the  subject  of  gravity  to-morrow. 


20 


MECHANICS. 


CONVERSATION  VII. 

OF  THE  ATTRACTION  OF  GRAVITATION. 

E.  Has  the  attraction  of  gravitation,  papa,  the 
same  eflPect  on  all  bodies,  whatever  be  their  distance 
from  the  earth  1 

F.  No  ;  this,  like  every  power  which  proceeds  from 
a  centre,  decreases  as  the  squares  of  the  distances  from 
that  centre  increase. 

E.  J  fear  that  I  shall  not  understand  this  unless  you 
illustrate  it  by  examples. 

F,  S appose  you  are  reading  at  the  distance  of  one 
foot  from  a  candle,  and  that  you  receive  a  certain 
quantity  of  light  on  your  book  ;  now  if  you  remove  to 
the  distance  of  two  feet  from  the  candle,  you  will,  by 
this  law,  enjoy  four  times  less  light  than  you  had  be- 
fore ;  here  then  though  you  have  increased  your  dis- 
tance but  two-fold,  yet  the  light  is  diminished  four- 
fold, because  four  is  the  square  of  two,  or  two  multi- 
plied by  itself.  If,  instead  of  removing  two  feet  from 
the  candle,  you  take  your  station  at  3,  4,  5,  or  6  feet 
distance,  you  will  then  receive,  at  the  different  dis- 
tances, 9,  16,  25,  36  times  less  light  than  when  yoi. 
were  within  a  single  foot  from  the  candle,  for  these,  as 
you  know,  are  the  squares  of  the  numbers,  3,  4,  5  and 
6.  The  same  is  applicable  to  the  heat  imparted  by  a 
fire  ;  at  the  distance  of  one  yard  from  which,  a  per- 
son will  enjoy  four  times  as  much  heat,  as  he  who  sits 
or  stands  two  yards  from  it ;  and  nine  times  as  much 
as  one  that  shall  be  removed  to  the  distance  of  three 
yards. 

C.  Is  then  the  attraction  of  gravity  four  times 
less  at  a  yard  distance  from  the  earth,  than  it  is  at  the 
surface  1. 

F,  No  ;  whatever  be  the  cause  of  attraction,  winch 
to  this  day  remains  undiscovered,  it  acts  from  the 
centre  of  the  earth,  and  not  from  its  surface,  and  hence 
the  difference  of  the  power  of  gravity  cannot  be  dis- 


ATTRACTION  OF  GRAVITATION.  21 

cerned  at  the  small  distances  to  which  we  can  have 
access  ;  for  a  mile  or  two,  which  is  much  higher  than, 
in  general,  we  have  opportunities  of  making  experi- 
ments, is  nothing  in  comparison  of  4000  miles,  the 
distance  of  the  centre  from  the  surface  of  the  earth. 
But  could  we  ascend  4000  miles  above  the  earth,  and 
of  course  be  double  the  distance  that  we  now  are  from 
the  centre,  we  should  there  find  that  the  attractive 
force  would  be  but  one-fourth  of  what  it  is  here  ;  or 
in  other  words  that  a  body,  which  at  the  surface  of  the 
earth  weighs  one  pound,  and,  by  the  force  of  gravity, 
falls  through  sixteen  feet  in  a  second  of  time,  would 
at  4000  mile^  above  the  earth  weigh  but  a  quarter  of 
a  pound,  and  fall  through  only  four  feet  in  a  second  * 

E.  How  is  that  known,  papa,  for  nobody  ever  was 
there  ? 

F.  You  are  right,  my  dear,  for  Garnerin,  who  some 
years  ago  astonished  all  the  people  of  the  metropolis 
and  its  neighbourhood,  by  his  flight  in  a  balloon, 
ascended  but  a  little  way  in  comparison  of  the  distance 
that  we  are  speaking  of.  However,  I  will  try  to  ex- 
plain in  what  manner  philosophers  have  come  by  their 
knowledge  on  this  subject. 

The  moon  is  a  heavy  body  connected  with  the  earth 
by  this  bond  of  attraction  ;  and  by  the  most  accurate 
observations  it  is  known  to  be  obedient  to  the  same 
laws  as  other  heavy  bodies  are:  its  distance  is  also 
clearly  ascertained,  being  about  240,000  miles,  or 
equal  to  about  sixty  semi-diameters  of  the  earth,  and  of 
course  the  earth's  attraction  upon  the  moon  ought  to 
diminish  in  the  proportion  of  the  square  of  this  dis- 

*  Ex.  Suppose  it  were  required  to  find  the  weight  of 
a  leaden  ball  at  the  top  of  a  mountain  three  miles  high, 
which,  on  the  surface  of  the  earth  weighs  20lb. 

If  the  semi-diameter  of  the  earth  be  taken  at  4000, 
then  add  to  this  the  height  of  the  mountain,  and  say,  as 
the  square  of  4003  is  to  the  square  of  4000,  so  is  20lb.  to  a 
fourth  proportional :  or  as  16024009  : 16000000  : :  20  :  19'97  : 
or  something  more  than  19lb.  15^03.,  which  is  the  weight 
of  the  leaden  ball  at  the  top  of  the  mountain. 


22  MECHANICS, 
tance ;  that  is,  it  ought  to  be  60  times  60,  or  3600 
times  less  at  the  moon  than  it  is  at  the  surface  of  the 
earth.    This  is  found  to  be  the  case,  by  the  measure 
of  the  deviation  of  its  orbit  from  a  right  line. 

Again,  the  earth  is  not  a  perfect  sphere,  but  a 
spheroid,  that  is,  of  the  shape  of  an  orange,  rather  flat 
at  the  two  ends  called  the  poles,  and  the  distance  from 
the  centre  to  the  poles  is  about  seventeen  or  eighteen 
miles  less  than  its  distance  from  the  centre  to  the  equa- 
tor ;  consequently,  bodies  ought  to  be  something  hea- 
vier at,  and  near  the  poles,  than  they  are  at  the  equa- 
tor, which  is  also  found  to  be  the  case.  Hence  it  is 
inferred,  that  the  attraction  of  gravitation  vanes  at  all 
distances  from  the  centre  of  the  earth,  in  proportion 
as  the  squares  of  those  distances  increase.* 

C.  It  seems  very  surprising  that  philosophers,  who 
have  discovered  so  many  things,  have  not  been  able 
to  find  out  the  cause  of  gravity.  Had  Sir  Isaac  New- 
ton been  asked  why  a  marble,  dropped  from  the  hand, 
falls  to  the  ground,  could  he  not  have  assigned  the 
reason  ? 

F.  That  great  man,  probably  the  greatest  man  that 
ever  adorned  this  world,  was  as  modest  as  he  was 
great,  and  he  would  have  told  you  he  knew  not  the 
cause. 

The  late  learned  Dr.  Price,  in  a  work  which  he 
published  forty-five  years  ago,  asks,  "  who  does  not 
remember  a  time  when  he  would  have  wondered  at 
the  question,  why  does  ivater  run  down  hill?  Uhat 
io-norant  man  is'  there  who  is  not  persuaded  that  he 
understands  this  perfectly  1  But  every  improved  nian 
knows  it  to  be  a  question  he  cannot  answer.  For 
the  descent  of  water,  like  that  of  other  heavy  bodies, 
depends  upon  the  attraction  of  gravitation,  the  cause 
of  which  is  still  involved  in  darkness. 

E.  You  just  now  said  that  heavy  bodies  by  the 
force  of  gravity  fall  sixteen  feet  in  a  second  of  time ; 
is  that  always  the  case  ? 

F.  Yes,  all  bodies  near  the  surface  of  the  earth  full 

*  See  Conver.  VI.  on  Astronomy. 


ATTRACTION  OF  GRAVITATION.  23 
at  that  rate  in  the  first  second  of  time,  but  as  the  attrac- 
tion of  gravitation  is  continually  acting,  so  the  velocity 
of  falling  bodies  is  an  increasing,  or,  as  it  is  usually 
called,  an  accelerating  velocity.  It  is  found,  by  very 
accurate  experiments,  that  a  body,  descending  from  a 
considerable  height  by  the  force  of  gravity,  falls  16 
feet  in  the  first  second  of  time  ;  3  times  16  feet  in  the 
next ;  5  times  16  feet  in  the  third  ;  7  times  16  feet  in 
the  fourth  second  of  time ;  and  so  on,  continually  in- 
creasing according  to  the  odd  numbers,  1,  3,  5,  7,  9, 
11,  &c.  In  our  latitude  the  true  distance  fallen  is 
16  feet  one-tvi^elfth  ;  but,  by  reason  of  the  centrifugal 
force,  this  distance  varies  a  little  in  different  latitudes. 
But  this  shall  be  explained  to  you  hereafter. 

CONVERSATION  VIII. 

OF  THE  ATTRACTION  OF  GRAVITATION. 

E,  Would  a  ball  of  twenty  pounds'  weight  here, 
weigh  half  an  ounce  less  on  the  top  of  a  mountain 
three  miles  high  ? 

F,  Certainly  ;  but  you  would  not  be  able  to  as- 
certain it  by  means  of  a  pair  of  scales  and  another 
weight,  because  both  weights  being  in  similar  situa- 
tions would  lose  equal  portions  of  their  gravity. 

E.  How,  then,  would  you  make  the  experiment  ? 

F.  By  means  of  one  of  those  stee'l  spiral-spring  in- 
struments which  you  have  seen  occasionally  used,  the 
fact  might  be  ascertained. 

C.  I  think,  from  what  you  told  us  yesterday,  that 
with  the  assistance  of  your  stop-watch,  I  could  tell 
the  height  of  any  place,  by  observing  the  number 
of  seconds  that  a  marble  or  other  heavy  body  would 
take  in  falling  from  that  height. 

F,  How  would  you  perform  the  calculation  1 
C.  I  should  go  through  the  multiplications  accord- 
ing to  the  number  of  seconds,  and  then  add  them 
together. 


24  MECHANICS. 

F.  Explain  yourself  more  particularly : — supposing 
you  were  to  let  a  marble  or  penny-piece  fall  down 
that  deep  well  which  we  saw  last  summer  in  the  brick 
field  near  Ramsgate,  and  that  it  was  exactly  five 
seconds  in  the  descent,  what  would  be  the  depth  of 
the  well  ? 

C.  In  the  first  second  it  would  fall  16  feet ;  in  the 
next  3  times  16  or  48  feet ;  in  the  third  5  times  16  or 
80  feet ;  in  the  fourth  7  times  16  or  112  feet ;  and  in 
the  fifth  second  9  times  16  or  144  feet :  now  if  I  add 
16,  48,  80,  112,  and  144  together,  the  sum  will  be 
400  feet,  which  according  to  your  rule  is  the  depth  of 
the  well.    But  was  the  well  so  deep  1 

F.  I  do  not  think  it  was,  but  we  did  not  make  the 
experiment ;  should  we  ever  go  to  that  place  again 
you  may  satisfy  your  curiosity.  You  recollect  that 
at  Dover  Castle  we  were  told  of  a  well  there  360  feet 
deep. 

Though  your  calculation  was  accurate,  yet  it  was 
not  done  as  nature  effects  her  operations ;  it  was  not 
performed  in  the  shortest  way. 

C.  I  should  be  pleased  to  know  an  easier  method  ; 
this  however  is  very  simple,  it  required  nothing  but 
multiplication  and  addition. 

F,  True,  but  suppose  I  had  given  you  an  exam- 
ple in  which  the  number  of  seconds  had  been  fifty  in- 
stead of  five,  the  work  would  have  taken  you  an  hour 
or  more  to  have  performed ;  whereas,  by  the  rule 
which  I  am  going  to  give,  it  might  have  been  done  in 
half  a  minute. 

C.  Pray  let  me  have  it ;  I  hope  it  will  be  easily 
remembered. 

F.  It  will ;  I  think  it  cannot  be  forgotten  after  it 
IS  once  understood.  The  rule  is  this,  *'  the  spaces 
described  by  a  body  falling  freely  from  a  state  of  rest 
increase  as  the  squares  of  the  times  increase."  Conse- 
quently you  have  only  to  square  the  number  of 
seconds,  that  is,  you  know,  to  multiply  the  number  into 
itself,  and  then  multiply  that  again  by  sixteen  feet, 


ATTRACTION  OF  GRAVITATION.  25 
Use  space  which  it  describes  in  the  first  second,  and 
you  have  the  required  answer.  Now  try  the  example 
of  the  welL 

C.  The  square  of  5,  for  the  time,  is  25,  which  mul- 
tiphed  by  16  gives  400,  just  as  I  brought  it  out  before. 
Now  if  the  seconds  had  been  50,  the  answer  would 
be  50  times  50,  which  is  2500,  and  this  multiplied  by 
16,  gives  40,000  for  the  space  required. 

F.  I  will  now  ask  your  sister  a  question,  to  try  how 
she  has  understood  this  subject.  Suppose  you  observe 
by  this  watch  that  tire  time  of  the  flight  of  your  bro- 
ther's arrow  is  exactly  six  seconds,  to  what  height  does 
it  rise  ? 

E.  This  is  a  different  question,  because  here  the 
ascent  as  well  as  the  fall  of  the  arrow  is  to  be  con-* 
sidered. 

F.  But  you  will  remember  that  the  time  of  the 
ascent  is  always  equal  to  that  of  the  descent ;  for  as 
the  velocity  of  the  descent  is  generated  by  the  force  of 
gravity,  so  is  the  velocity  of  the  ascent  destroyed  by 
the  same  force. 

E.  Then  the  arrow  was  three  seconds  only  m  fall- 
ing ;  now  the  square  of  3  is  9,  which  multiplied  by 
16,  for  the  number  of  feet  described  in  the  first  second, 
is  equal  to  144  feet,  the  height  to  which  it  rose. 

F,  Now,  Charles,  if  I  get  you  a  bow  which  will 
carry  an  arrow  so  high  as  to  be  fourteen  seconds 
in  its  flight,  can  you  tell  me  the  height  to  which  it 
ascends  ?  ... 

C.  I  can  now  answer  you  without  hesitation  ; — it 
will  be  7  seconds  in  falling,  the  square  of  which  is  49, 
and  this  again  multiplied  by  16  will  give  784  feet,  or 
rather  more  than  261  yards,  for  the  answer. 

F.  If  you  will  now  consider  the  example  which  you 
did  the  long  way,  you  will  see  that  the  rule  which  I 
have  given  you  answers  very  completely.  In  the  first 
second  the  body  fell  16  feet,  and  in  the  next  48,  these 
added  together  make  64,  which  is  the  square  of  the  2 
seconds  muhiplied  by  16.  The  same  holds  true  oiUm 
3  first  seconds,  for  in  the  third  second  it  fell  80  iect. 
C 


26  MECHANICS. 

whicli  added  to  the  64,  give  144,  equal  to  the  square 
of  3  multiplied  by  16.  Again,  in  the  fourth  second  it 
fell  112  feet,  which  added  to  144,  give  256,  equal  to 
the  square  of  4  multiplied  by  16  ;  and  in  the  fifth 
second  it  fell  144  feet,  which  added  to  256,  give  400, 
equal  to  the  square  of  5  multiplied  by  16.  Thus  you 
will  find  the  rule  holds  in  all  cases,  that  the  spaces 
described  by  bodies  falling  fvpelii  from  a  state  of  rest 
increase  os  the  squares  of  the  times  increase. 

C.  I  think  I  shall  not  forget  the  rule.  I  will  also 
shew  my  cousin  Henry  how  he  may  know  the  height 
to  which  his  bow  will  carry. 

F.  The  surest  way  of  keeping  what  knowledge  we 
have  obtained,  is  by  communicating  it  to  our  friends. 

C.  It  is  a  very  pleasant  circumstance  indeed,  that 
the  giving  away  is  the  best  method  of  keeping,  for  1 
am  sure  the  being  able  to  oblige  one's  friends  is  a 
most  delightful  thing. 

F.  Your  sentiments  are  highly  gratifying  to  me ; 
fain  would  I  confirm  them  by  adding  to  your  stock  of 
knowledge.  And,  in  reference  to  this  subject,. it  may 
be  necessary  to  guard  you  against  the  notion,  that  be- 
cause the  spaces  described  by  falling  bodies  are  as  the 
square^  of  the  times,  the  velocities  increase  in  the  same 
ratio.  This  is  not  the  case.  The  velocity  acquired 
by  a  body  falling  freely,  at  the  end  of  the  first  second 
of  its  motion,  is  such  as,  if  it  continued  uniform, 
would  carry  it  over  32  feet  in  the  next  second. 
And  in  all  succeeding  intervals  the  velocities  are  as 
the  times:  that  is,  at  the  end  of  2,  3,  4,  and  5 
seconds,  the  velocities  acquired  will  be  respectively, 
twice,  thrice,  four  times,  and  five  times  32  feet ;  or, 
64,  96,  128,  and  160  feet. 

E,  Before  we  quit  this  part  of  the  subject,  papa, 
let  me  try  if  I  thoTouohly  comprehend  you.  A  fall- 
ing body  having  been  in  motion  4  seconds,  will  have 
descended  256^feet,  and  will  then  have  a  velocity  of 
128  feet ;  but  the  motion  still  accelerates  and  causes 
tlie  body  to  pass  over  nine  times  16,  or  144  feet,  m 
the  5th  'second,  making  in  all  400  feet :  it  will  then 


CENTRE  OF  GRAVITY.  27 
have  acquired  a  velocity  of  5  times  32,  or  160  feet  in 
a  second,  which  if  it  continued  uniform  for  another  5 
seconds,  would  carry  the  body  over  800  feet,  or  just 
twice  the  space  described  by  the  body  in  the  first  5 
seconds,  during  which  its  motion  was  equably  accele- 
rated by  gravity. 

F.  You  have  most  accurately  caught  the  distmc- 
tion  I  wished  you  to  understand. 

With  this  we  conclude  our  present  conversation. 


CONVEKSATION  IX. 

ON  THE  CENTRE  OF  GRAVITY. 

F.  We  are  now  going  to  treat  upon  -the  Cenfre  of 
Gravity,  which  is  that  point  of  a  body  in  which  its 
whole  weight  is  as  it  were  concentrated,  and  upon 
which,  if  the  body  be  freely  suspended,  it  will  rest ; 
;  and  in  all  other  positions  it  will  endeavour  to  descend 
'  to  the  lowest  place  to  which  it  can  get. 

C.  All  bodies  then,  of  whatever  shape,  have  a  cen- 
I  tre  of  gravity? 

F.  They  have  ;  and  if  you  conceive  a  line  drawn 
i'  from  the  centre  of  gravity  of  a  body  towards  the  cen- 
'  tre  of  the  earth,  that  line  is  called  the  line  of  direction, 
\  alono-  which  every  body,  not  supported,  endeavours 
!  to  fall.  If  the  line  of  direction  fall  within  the  base  of 
I  any  body,  it  will  stand  ;  but  if  it  does  not  fall  within 

the  base,  the  body  will  fall. 
I     If  I  place  the  piece  of  wood  a  on  the  edge 
!  of  a  table,  and  from  a  pin  c  at  its  centre  of 
gravity  be  hung  a  little  weight  d,  the  line 
'  of  direction  cd  falls  within  the  base,  and 
therefore,  though  the  wood  leans,   yet  it 
stands  secure.    But  if  upon  a  another  piece 
I  of  wood  b  be  placed,  it  is  evident  that  the  centre  ot 
-i  gravity  of  the  whole  will  be  now  raised  to  e,  at  which 
I  point  if  a  weight  be  hung,  it  will  be  found  that  the 
line  of  direction  falls  out  of  the  base,  and  therefore 
i  the  body  must  fall. 


28  MECHANICS. 
.  E.  I  think  I  now  see  the  reason  of  the  advice 
wkich  you  gave  me,  when  we  w^cre  going  across  the 
Thames  in  a  boat. 

F,  I  told  you  that  if  ever  you  were  overtaken  by  a 
storm,  or  by  a  squall  of  wind,  while  you  were  on  the 
water,  never  to  let  your  fears  so  get  the  better  of  you 
as  to  make  you  rise  from  your  seat,  because,  by  so 
doing,  you  would  elevate  the  centre  of  gravity,  and 
thereby,  as  is  evident  by  the  last  experiment,  increase 
the  danger :  whereas,  if  all  the  persons  in  the  vessel 
were,  at  the  moment  of  danger,  instantly  to  slip  from 
their  places  on  to  the  bottom,  the  risk  would  be  ex- 
ceedingly diminished,  by  bringing  the  centre  of  gra- 
vity much  lower  within  the  vessel.  The  same  prmci- 
ple  is  applicable  to  those  who  may  be  in  danger  of 
being  overturned  in  any  carriage  whatever. 

E.  Surely  then,  papa,  those  stages  which  load  their 
tops  with  a  dozen  or  more  people,  cannot  be  safe  for 
the  passengers. 

F.  They  are  very  unsafe  ;  but  they  would  be  more 
so  were  not  the  roads  about  the  metropolis  remarkably 
even  and  good ;  and,  in  general,  it  is  only  withm 
twenty  or  thirty  miles  of  London,  or  other  great  towns, 
that  the  tops  of  caaTiages  are  loaded  to  excess. 

C.  I  understand,  then,  that  the  nearer  the  centre 
of  gravity  is  to  the  base  of  a  body  the  firmer  it  will 
stand.  ,         -  , 

F.  Certainly  ;  and  hence  you  learn  the  reason  why 
conical  bodies  stand  so  sure  on  their  bases,  for,  the 
tops  being  small  in  comparison  of  the  lower  parts,  the 
centre  of  gravity  is  thrown  very  low  ;  and,  it  the  cone 
be  uprio  ht,  or  perpendicular,  the  line  of  direction  talis 
in  the  middle  of  the  base,  which  is  another  fundamen- 
tal property  of  steadiness  in  bodies.  For  the  broader 
the  base,  and  the  nearer  the  line  of  direction  is  to  the 
middle  of  it,  the  more  firmly  does  a  body  stand  ;  but 
if  the  hue  of  direction  fall  near  the  edge  the  body  is 
easily  overthrown.  ,  n  j 

C.  Is  that  the  reason  why  a  ball  is  so  easily  rolled 
along  a  horizontal  plane  1 


CENTRE  OF  GRAVITY.  29 
F.  It  IS ;  for  m  all  spherical  bodies  the  base  is  but 
a  point,  consequently  almost  the  smallest  force  is  snt- 
ficient  to  remove  the  line  of  direction  out  of  it.  Hence 
it  is  evident  that  heavy  bodies  situ- 
ated on  an  inclined  plane  v^^iil,  while 
the  line  of  direction  falls  within  the 
base,  slide  down  upon  the  plane; 
but  they  will  roll  when  that  line  falls 
without  the  base.  The  body  a  will 
slide  down  the  plane  de,  but  the  bo- 
dies b  and  c  will  roll  down  it. 

E,  I  have  seen  buildings  lean  very  much  out  of  a 
straight  line  ;  why  do  they  not  fall  1 

F,  It  does  not  follow,  because  a  building  leans,  that 
the  centre  of  gravity  does  not  fall  within  the  base. 
There  is  a  high  tower  at  Pisa,  a  town  in  Italy,  which 
leans  fifteen  feet  out  of  the  perpendicular ;  strangers 
tremble  to  pass  by  it ;  still  it  is  found  by  experiment 
that  the  line  of  direction  falls  within  the  base,  and 
therefore  it  will  stand  while  its  materials  hold  together. 

A  wall  at  Bridgenorth,  in  Shropshire,  which  I  have 
!  seen,  stands  in  a  similar  situation,  for  so  long  as  a 
I  line  cb,  let  fall  from  the  centre  of  gravity  c  of  the 
i  building*  ab,  passes  within  the  base  cd,  it  will  remain 
I  firm,  unless  the  materials  with  which  it  is  built  go  to 
{  decay. 

1  C.  It  must  be  of  great  use,  in  many  cases,  to  know 
\    the  method  of  finding  the  centre  of  gravity  in  different 

kinds  of  bodies. 

F.  There  are  many  easy  rules  for  this  with  respect 
I  to  all  manageable  bodies  :  I  will  mention  one,  which 
j  depends  on  the  property  which  the  centre  of  gravity 
I    has,  of  always  endeavouring  to  descend  to  the  lowest 

point. 

If  a  body  a  be  freely  suspended  on  ^ 
a  pin  6,  and  a  plumb  line  be  be  hung  by  -hj-jj   /  A 
the  same  pin,  it  will  pass  through  the  Yj. 
centre  of  gravity,  for  that  centre  is  not  ^^^^ 
jL  the  lowest  point,  till  it  fall  in  the  same  °' 

*  See  the  Vignette  to  this  volume. 


30 


MECHANICS. 


line  as  the  plumb  line.  Mark  the  line  he ;  then  hang 
the  body  up  by  any  other  point,  as  d,  with  the  plumb 
line  J/,  which  will  also  pass  through  the  centre  of 
gravity,  for  the  same  reason  as  before  ;  and  therefore, 
as  the  centre  of  gravity  is  somewhere  in  be,  and  also 
in  some  point  of  df,  it  must  be  in  the  point  e  where 
those  lines  cross. 

CONVERSATION  X. 

OF  THE  CENTRE  OF  GRAVITY. 

C.  How  do  those  people  who  have  to  load  carts 
and  waggons  with  light  goods,  as  hay,  wool,&c.  know 
where  to  find  the  centre  of  gravity  ? 

F.  Perhaps  the  generality  of  them  never  heard  of 
such  a  principle  ;  and  it  seems  surprising  that  they 
should  nevertheless  make  up  their  loads  with  such  ac- 
curacy as  to  keep  the  line  of  direction  in  or  near  the 
middle  of  the  base. 

E.  I  have  sometimes  trembled  to  pass  by  the  hop 
waggons  which  we  have  met  on  the  Kent  Koad. 

F.  And  without  any  impeachment  of  your  courage, 
for  they  are  loaded  to  such  an  enormous  height,  that 
they  totter  every  inch  of  the  road.  It  would  indeed 
be  impossible  for  one  of  these  to  pass  with  tolerable 
security  along  a  road  much  inclined  ;  the  centre  of 
gravity  being  removed  so  high  above  the  body  of  the 
carriage,  a  small  declination  on  one  side  or  the  other 
would  throw  the  line  of  direction  out  of  the  base. 

E.  When  brother  James  falls  about,  is  it  because 
he  cannot  keep  the  centre  of  gravity  between  his  feet  ? 

F.  That  is  the  precise  reason  why  any  person, 
whether  old  or  young,  falls.  And  hence  you  learn 
that  a  man  stands  much  firmer  with  his  feet  a  little 
apart  than  if  they  were  quite  close,  for  by  separating 
them  he  increases  the  base.  Hence  also  the  difiiculty 
of  sustaining  a  tall  body,  as  a  walking  cane,  upon  a 
narrow  foundation. 

E.  How  do  rope  and  wire  dancers,  whom  I  have 
seen  at  the  Circus,  manage  to  balance  themselves  ? 


CENTRE  OF  GRAVITY.  31 

F  They  generally  hold  a  long  pole,  with  weights 
at  each  end,  across  the  rope  on  which  they  dance, 
keepino-  their  eyes  fixed  on  some  object  parallel  to  the 
rope,  by  which  means  they  know  when  their  centre 
of  gravity  declines  to  one  side  of  the  rope  or  the  other, 
and  thus,  by  the  help  of  the  pole,  they  are  enabled  to 
keep  the  centre  of  gravity  over  the  base,  narrow  as  it 
is  It  is  not,  however,  rope-dancers  only  who  pay 
attention  to  this  principle,  but  the  most  common  ac- 
tions of  the  people  in  general  are  regulated  by  it. 

C.  In  what  respects'? 

F  We  bend  forward  when  we  go  up  stairs,  or  rise 
from*  our  chair,  for  when  we  are  sitting  our  centre  of 
gravity  is  on  the  seat,  and  the  line  of  direction  falls 
behind  our  base  ;  we  therefore  lean  forward  to  bring 
the  line  of  direction  towards  our  feet.  For  the  same 
reason  a  man  carrying  a  burthen  on  his  back  leans 
forward  ;  and  backward  if  he  carries  it  on  his  breast. 
If  the  load  be  placed  on  one  shoulder  he  leans  to  the 
other  If  we  slip  or  stumble  with  one  foot,  we  natu- 
rally extend  the  opposite  arm,  making  the  same  use  of 
it  as  the  rope-dancer  does  of  his  pole. 

This  property  of  the  centre  of  gravity  always  en- 
deavourincr  to  descend,  will  account  for  appearances, 
which  are^sometimes  exhibited  to  excite  the  surprise 
of  spectators. 

£.  What  are  those  papa? 

F  One  is,  that  of  a  double  cone,  appearing  to  roll 
up  two  inclined  planes,  forming  an  angle  with  each 
other  for  as  it  rolls  it  sinks  between  them,  and  by  that 
means  the  centre  of  gravity  is  actually  descending. 

Let  a  body  ej\  consisting  of 
two  equal  cones  united  at  their  o^^^^^^^^^i 

bases,  be  pla  ced  upon  the  edges  ^^r^;:^:^^—  ^ 

of  two  straight  smooth  rulers  /Q^^^^^^,,:^^...::^^^^^^ 

ah  and  cd,  which  at  one  end 

meet  in  an  angle  at  a,  and  rest  ^-^ 

on  a  horizontal  plane,  and  at  °  ,  i 

the  other  are  raised  a  little  above  the  p  ane  ;  the  body 

will  roll  towards  the  elevated  end  of  the  rulers,  and 


32  MECHANICS. 

appear  to  ascend  ;  the  parts  of  the  cone  that  rest  on 
the  rulers  growing  smaller  as  they  go  over  a  larger 
opening,  and  thus  letting  it  down,  the  centre  of  gra- 
vity descends.  But  you  must  remember  that  the 
height  of  the  planes  must  be  less  than  the  radius  of 
the  base  of  the  cone. 

C.  Is  it  upon  this  principle  that  a  cylinder  is  made 
to  roll  up  hill  ? 

F.  Yes  it  is,  but  this  can  be  effected  only  to  a  small 
distance.  If  a  cylinder  of  pasteboard,  or  very  light 
wood,  ah,  having  its  centre  of  gravity 
at  c,  be  placed  on  the  inclined  plane 
de,  it  will  roll  down  the  inclined 
plane,  because  a  line  of  direction 
from  that  centre  lies  out  of  the  base. 
If  1  now  fill  the  little  hole  o  with  a  Tig.  12. 
plug  of  lead,  it  will  roll  up  the  in- 
clined plane,  till  the  lead  gets  near  the  base,  where 
it  will  lie  still :  because  the  centre  of  gravity,  by 
means  of  the  lead,  is  removed  from  c  towards  the  plug, 
and  therefore  is  descending,  though  the  cylinder  is 
ascending. 

Before  I  put  an  end  to  this  subject,  I  will  shew 
you  another  experiment,  which  without  understanding 
the  principle  of  the  centre  of  gravity  cannot  be  ex- 
plained. Upon  this  stick  a,  which, 
of  itself  would  fall,  because  its  centre 
of  gravity  hangs  over  the  table  b,  I 
suspend  a  bucket  c,  fixing  another 
stick  d,  one  end  in  a  notch  between  a 
and  e,  and  the  other  against  the  in- 
side of  the  pail  at  the  bottom.  Now 
you  will  see  that  the  bucket  will,  in  this  position,  be 
supported,  though  filled  with  water.  For  the  bucket 
being  pushed  a  little  out  of  the  perpendicular,  by  the 
stick  d,  the  centre  of  gravity  of  the  whole  is  brought 
under  the  table,  and  consequently  supported  by  it. 

The  knowledge  of  the  principle  of  the  centre  of 
gravity  in  bodies,  will  enable  you  to  explain  the 
structure  of  a  variety  of  toys  which  are  put  into  the 


LAWS  OF  MOTION.  33 
hands  of  children,  such  as  the  little  sawyer,  rope- 
dancer,  tumbler,  4c. 


CONVERSATION  XI. 

ON  THE  LAWS  OF  MOTION. 

C.  Are  you  now  going,  papa,  to  describe  those  ma- 
chines, which  vou  call  mechanical  powers  ? 

F.  We  must,  I  believe,  defer  that  a  day  or  two 
longer,  as  I  have  a  few  more  general  prhiciples  with 
which  I  wish  you  previously  to  be  acquamted. 

E.  What  are  these,  papal 

F,  In  the  first  place,  you  must  well  understand 
what  are  denominated  the  three  general  laws  of  mo- 
tion :  the  first  of  which  is,  that  every  body  will  con- 
tinue  in  its  state  of  rest,  or  of  unform  motion,  until  it 
is  compelled  bu  some  force  to  change  its  state.  _  I  his 
constitutes  what  is  denominated  the  inertia,  or  macti;- 
vity  of  matter.  And  it  may  be  observed  that,  m  all 
cases,  the  quantity  of  motion  gained  by  one  body  is 
always  equal  to  that  lost  by  some  other  body. 

C.  There  is  no  difficulty  of  conceiving  that  a  body, 
as  this  ink-stand,  in  a  state  of  rest  must  always  remain 
so,  if  no  external  force  be  impressed  upon  it  to  give  it 
motion.  But  I  know  of  no  example  which  will  lead 
me  to  suppose,  that  a  body  once  put  into  motion 
would  of  itself  continue  so.  ,   .    i  i 

F  You  will,  I  think,  presently  admit  the  latter 
part  of  the  assertion  as  well  as  the  former,  although  it 
cannot  be  established  by  experiment.  ^ 

E.  I  shall  be  glad  to  hear  how  this  is. 

F.  You  will  not  deny  that  the  ball  which  you  strike 
from  the  trap  has  no  more  power  either  to  destroy 
its  motion,  or  cause  any  change  in  its  velocity,  than  it 
has  to  change  its  shape.  ■, 

C.  Certainly  ;  nevertheless,  m  a  few  seconds  after 
I  have  struck  the  ball  with  all  my  force,  it  falls  to 
the  ground,  and  then  stops.  _ 

F  Do  YOU  find  no  diflference  in  the  time  that  is 
^  C  2 


31 


MECHANICS. 


taken  up  before  it  conies  to  rest,  even  supposing  your 
blow  the  same  1 

C.  Yes,  if  1  am  playing  on  the  grass,  it  rolls  to  a 
less  distance  than  when  1  play  on  the  smooth  gravel. 

F.  You  fmd  a  like  difference  when  you  are  playing 
at  marbles,  if  you  play  in  the  gravel  court,  or  on  the 
even  pavement  in  the  arcade. 

C.  The  marbles  run  so  easily  on  the  smooth  stones 
in  the  arcade,  that  we  can  scarcely  shoot  with  a  force 
small  enough. 

E.  And  I  remember  Charles  and  my  cousin  were 
last  winter  trying  how  far  they  could  shoot  their 
marbles  along  the  ice  in  the  canal ;  and  they  went  a 
prodigious  distance,  in  comparison  of  that  which  they 
would  have  gone  on  the  gravel,  or  even  on  the  pave- 
ment in  the  arcade. 

F.  Now  these  instances  properly  applied  will  con- 
vince you,  that  a  body  once  put  into  motion  would 
go  on  for  ever,  if  it  were  not  compelled  by  some  ex- 
ternal force  to  change  its  state. 

C.  I  perceive  what  you  are  going  to  say  : — it  is  the 
rubbing  or  friction  of  the  marbles  against  the  ground 
which  does  the  business.  For  on  the  pavement  there 
are  fewer  obstacles  than  on  the  gravel,  and  fewer  on 
the  ice  than  on  the  pavement ;  and  hence  you  would 
lead  us  to  conclude,  that  if  all  obstacles  were  removed, 
they  might  proceed  on  for  ever.  15ut  what  are  we  to 
say  of  the  ball  ;  what  stops  that  ? 

F.  Besides  friction,  there  is  another  and  still  more 
important  circumstance  to  be  taken  into  consideration, 
wiiich  affects  the  ball,  marbles,  and  every  body  in 
motion. 

C.  I  understand  you,  that  is  the  action  of  gravitation, 
F.  It  is ;  for  from  what  we  said  when  we  con- 
versed on  that  subject,  it  appeared  that  gravity  has  a 
tendency  to  bring  every  body  in  motion  to  the  earth  ; 
con.se(|uently,  in  a  few  seconds,  your  ball  must  come 
to  the  ground  by  that  cause  alone  ;  but  besides  the  at- 
traction of  gravitation,  there  is  the  resistance  which 
the  air,  through  which  the  ball  moves,  makes  to  its 
passage . 


LAWS  OF  MOTION. 


E.  That  cannot  be  much,  I  think. 

F.  Perhaps,  with  regard  to  the  ball  struck  from 
your  brother's  trap,  it  is  of  no  great  consideration,  be- 
cause the  velocity  is  but  small ;  but  in  all  great  velo- 
cities, as  that  of  a  ball  from  a  musket  or  cannon,  there 
will  be  a  material  difference  between  the  theory  and 
practice,  if  it  be  neglected  in  the  calculation.  Move 
your  mamma's  riding-whip  through  the  air  slowly,  and 
you  observe  nothing  to  remind  you  that  there  is  this 
resisting  medium  ;  but  if  you  swing  it  with  consider- 
able swiftness,  the  noise  which  it  occasions  will  in- 
form you  of  the  resistance  it  meets  with  from  some- 
thing, which  is  the  atmosphere. 

C.  If  I  now  understand  you,  the  force  which  com- 
pels a  body  in  motion  to  stop,  is  of  three  kinds  ;  1.  the 
attraction  of  gravitation  ; — 2.  the  resistance  of  the  air ; 
— and  3.  the  resistance  it  meets  with  from  friction. 

F.  You  are  quite  right. 

C.  I  have  now  no  difficulty  of  conceiving,  that  a 
body  in  motion  will  not  come  to  a  state  of  rest,  till  it 
is  brought  to  it  by  an  external  force,  acting  upon  it  in 
someway  or  other.  I  have  seen  a  gentleman,  when 
skating  on  very  slippery  ice,  go  a  great  way  without 
any  exertion  to  himself,  but  where  the  ice  was  rough 
be  could  not  go  half  the  distance  without  making 
fresh  efforts. 

F.  I  will  mention  another  instance  or  two  of  this 
law  of  motion.  Put  a  basin  of  water  into  your  little 
sister's  waggon,  and  when  the  water  is  perfectly  stdl 
move  the  waggon,  and  the  water,  resisting  the  motion 
of  the  vessel,  will  at  first  rise  up  in  the  direction  con- 
trary to  that  in  which  the  vessel  moves.  If,  when  the 
motion  of  the  vessel  is  communicated  to  the  water,  you 
suddenly  stop  the  waggon,  the  water,  in  endeavouring 
to  continue  the  state  of  motion,  rises  up  on  the  oppo- 
site side. 

In  like  manner,  if,  while  you  are  sitting  quietly  on 
your  horse,  the  animal  starts  forward,  you  will  be  in 
danger  of  falling  off  backward  ;  but  if,  while  you  are 
galloping  along,  the  animal  stops  on  a  sudden,  you 
will  be  liable  to  be  thrown  forward. 


36 


MECHANICS. 


C.  This  I  know  by  experience,  but  I  was  not  aware 
of  the  reason  of  it  till  to-day. 

F.  One  of  the  first,  and  not  least  important,  uses 
of  the  principles  of  natural  philosophy  is,  that  they 
may  be  applied  to,  and  will  explain,,  many  of  the 
common  concerns  of  life. 

We  now  come  to  the  second  law  of  motion ;  which 
is,  ''that  the  change  of  motion  is  proportional  to  the 
Jorce  impressed,  and  in  the  direction  of  that  force, ^' 

C.  There  is  no  difficulty  in  this ;  for  if,  while  my 
cricket-ball  is  rolling  along,  after  Henry  has  struck 
it,  I  strike  it  again,  it  goes  on  with  increased  velo- 
city, and  that  in  proportion  to  the  strength  which  I 
exert  on  the  occp.sion  ;  whereas,  if,  while  it  is  rolling, 
1  strike  it  back  again,  or  give  it  a  side  blow,  I  change 
the  direction  of  its  course. 

F.  In  the  same  way,  gravity,  and  the  resistance  of 
the  atmosphere,  change  the  direction  of  a  cannon-ball 
from  its  course  in  a  straight  line,  and  bring  it  to  the 
ground  ;  and  the  ball  goes  to  a  farther  or  less  distance, 
in  proportion  to  the  quantity  of  powder  used. 

The  third  law  of  motion  is,  "  that,  to  every  action 
of  one  body  upon  another,  there  is  an  equal  and  con^ 
trary  re-action."  If  1  strike  tliis  table,  I  communi- 
cate to  it  (which  you  perceive  by  the  shaking  of  the 
glasses)  the  motion  of  my  hand :  and  the  table  re- 
acts against  my  hand,  just  as  much  as  my  hand  acts 
against  the  table. 

^  If  you  press  with  your  finger  one  scale  of  a  balance, 
to  keep  it  in  equilibrio  with  a  pound  weight  in  the 
other  scale,  you  will  perceive  that  the  scale  pressed 
by  the  finger  acts  against  it  with  a  force  equal  to  a 
pound,  with  v.'hich  the  other  scale  endeavours  to 
descend.  In  all  cases,  the  quantity  of  motion  gained 
by  one  body  is  always  equal  to  that  lost  by  the  other 
in  the  same  direction.  Thus,  if  a  ball  in  motion 
strike  another  at  rest,  the  motion  communicated  to 
the  latter  will  be  taken  from  the  former,  and  the  v  elo- 
city  of  tiie  former  will  be  proportionally  diminished. 


LAWS  OF  MOTION.  37 

A  horse  drawing  a  heavy  load,  is  as  much  drawn 
back  by  the  load  as  he  draws  it  forward. 

E.  1  do  not  comprehend  how  the  cart  draws  the 

^^^F!'But  the  progress  of  the  horse  is  impeded  by  the 
joad,  which  is  the  same  thing;  for  the  force  which 
the  horse  exerts  would  carry  him  to  a  greater  distance 
in  the  same  time,  were  he  freed  from  the  mcumbrance 
of  the  load,  and,  therefore,  as  much  as  his  progress 
falls  short  of  that  distance,  so  much  is  he,  in  eliect, 
drawn  back  by  the  re-action  of  the  loaded  cart. 

Ao-ain,  if  you  and  your  brother  were  in  a  boat,  and 
if  by  means  of  a  rope,  you  were  to  attempt  to  draw 
another  to  you,  the  boat  in  which  you  were  would  be 
as  much  pulled  toward  the  empty  boat  as  that  would 
be  moved  to  you  ;  and,  if  the  weights  of  the  two  boats 
were  equal,  they  would  meet  in  a  point  halt  way  be- 
tween the  two.  . 

If  you  strike  a  glass  bottle  with  an  iron  hammer,  • 
the  blow  will  be  received  by  the  hammer  and  the 
fvlass  ;  and  it  is  immaterial  whether  the  hammer  be 
moved  against  the  bottle  at  rest,  or  the  bottle  be 
moved  against  the  hammer  at  rest,  yet  the  bottle  will 
be  broken,  though  the  hammer  be  not  injured,  because 
the  same  blow  which  is  sufficient  to  break  glass  is 
not  sufficient  to  break  or  injure  a  lump  of  iron. 

From  this  law  of  motion  you  may  learn  m  what 
manner  a  bird,  by  the  stroke  of  its  wmgs,  is  able  to 
support  the  weight  of  its  body. 
C.  Pray  explain  this,  papa. 

F  If  the  force  with  which  it  strikes  the  air  below 
it  is  equal  to  the  weight  of  its  body,  then  the  re-action 
of  the  air  upwards  is  likewise  equal  to  it ;  and  the  bird, 
beinp;  acted  upon  by  two  equal  forces  in  contrary  di- 
rections, will  rest  between  them.  If  the  force  ot  the 
stroke  is  greater  than  its  weight,  the  bird  will  rise 
with  the  difference  of  these  two  forces ;  and,  it  the 
stroke  be  less  than  its  weight,  then  it  will  sink  with 
the  difference. 


38 


MECHANICS. 


CONVERSATION  XII. 

ON   THE   LAWS   OF  MOTION. 

C.  Are  those  laws  of  motion  which  you  explained 
yesterday  of  great  importance  in  natural  philosophy  1 

F.  Yes,  they  are,  and  should  be  carefully  commit- 
ted to  memory.  They  were  assumed  by  Sir  I.  New- 
ton as  the  fundamental  principles  of  mechanics,  and 
you  will  find  them  at  the  head  of  most  books  writ- 
ten on  these  subjects.  From  these  also  we  are  natu- 
rally led  to  some  other  branches  of  science,  which, 
though  we  can  but  slightly  mention,  should  not  be 
wholly  neglected.  They  are,  in  fact,  but  corollaries 
to  the  laws  of  motion. 

E.  What  is  a  corollary,  papa? 

F.  It  is  nothing  more  than  some  truth  clearly  de- 
ducible  from  some  other  truth  before  demonstrated  or 
admitted.  Thus  by  the  first  law  of  motion  every  body 
must  endeavour  to  continue  in  the  state  into  which  it  is 
put,  whether  it  be  of  rest,  or  uniform  motion  in  a  straight 
line  :  from  which  it  follows,  as  a  corollary,  that  when 
we  see  a  body  move  in  a  curve  line,  it  must  be  acted 
upon  by  at  least  two  forces. 

C.  When  I  whirl  a  stone  round  in  a  sling,  what 
are  the  two  forces  which  act  upon  the  stone  1 

F.  There  is  the  force  by  which,  if  you  let  go  the 
string,  the  stone  will  fly  off  in  a  right  line  ;  and  there 
is  the  force  of  the  hand,  which  keeps  it  in  a  circular 
motion. 

E.  Are  there  any  of  these  circular  motions  in 
nature  ? 

F,  The  moon  and  all  the  planets  move  by  this 
law  : — to  take  the  moon  as  an  instance.  It  has  a 
constant  tendency  to  the  earth,  by  the  attraction  of 
gravitation,  and  it  has  also  a  tendency  to  proceed  in  a 
right  line,  by  that  projectile  force  impressed  upon  it 
by  the  Creator,  in  the  same  manner  as  the  stone  flies 
from  your  hand  ;  now,  by  the  joint  action  of  these 
two  forces  it  describes  a  circular  motion. 


LAWS  OF  MOTION. 


39 


E.  And  what  would  be  the  consequence,  supposing 
the  projectile  force  to  cease  ? 

F.  The  moon  must  fall  to  the  earth  ;  and  if  the 
force  of  gravity  were  to  cease  acting  upon  the  moon, 
it  would  fly  off  into  infinite  space.  Now  the  projectile 
force,  when  applied  to  the  planets,  is  called  the  cen- 
trifugal force,  as  having  a  tendency  to  recede  or  fly 
from  the  centre  ;  and  the  other  force  is  termed  the 
ceiitripelal  force,  from  its  tendency  to  some  point  as  a 
centre. 

C.  And  all  this  is  in  consequence  of  the  inactivity 
of  matter,  by  which  bodies  have  a  tendency  to  con- 
tinue in  the  same  state  they  are  in,  whether  of  rest  or 
motion  ? 

F,  You  are  right,  and  this  principle,  which  Sir 
Isaac  Newton  assumed  to  be  in  all  bodies,  he  called 
their  vis  ineriice,  which  has  been  referred  to  before. 

C.  A  few  mornings  ago  you  shewed  us  that  the 
attraction  of  the  earth  upon  the  moon*  is  3600  times 
less,  than  it  is  upon  heavy  bodies  near  the  earth's  sur- 
face. Now  as  this  attraction  is  measured  by  the  space 
fallen  through  in  a  given  time,  I  have  endeavoured  to 
calculate  the  space  which  the  moon  would  fall  through 
in  a  minute,  were  the  projectile  force  to  cease. 

F,  Well,  and  how  have  you  brought  it  out  ? 

C.  A  body  falls  here  16  feet  in  the  first  second, 
consequently  in  a  minute,  or  60  seconds,  it  would  fall 
60  times  60  feet,  multiplied  by  16,  that  is  3600  feet, 
which  is  to  be  multiplied  by  16  ;  and  as  the  moon 
would  fall  through  3600  times  less  space  in  a  given 
time  than  a  body  here,  it  would  fall  only  16  feet  in 
the  first  minute. 

F.  Your  calculation  is  accurate.  I  will  recall  to 
your  mind  the  second  law,  by  which  it  appears,  that 
every  motion,  or  change  of  motion,  produced  in  a  body, 
7nust  be  proportional  to,  and  in  the  direction  of,  the 
force  impressed.  Therefore,  if  a  moving  body  receives 
an  impulse  in  the  direction  of  its  motion,  its  velocity 

*  See  Conversation  IV. 


40 


MECHANICS. 


vviil  be  increased  ; — if  in  the  contrary  direction,  its 
velocity  will  be  diminished  ; — but  if  the  force  be  im- 
piessed  in  a  direction  oblique  to  that  in  which  it  moves, 
then  its  direction  will  be  between  that  of  its  former 
motion,  and  that  of  the  new  force  impressed. 

C.  This  I  know  from  the  observations  I  have  made 
with  my  cricket-ball. 

F,  By  this  second  law  of  motion,  you  will  easily 
understand,  that  if  a  body  at  rest  receive  two  im- 
pulses at  the  same  time,  from  forces  whose  directions 
do  not  coincide,  it  will,  by  their  joint  action,  be  made 
to  move  in  a  line  that  lies  between  the  direction  of 
the  forces  impressed. 

E.  Have  you  any  machine  to  prove  this  satisfac- 
torily to  the  senses  ? 

F.  There  are  many  such  invented  by  different  per- 
sons, descriptions  of  which  you  will  hereafter  find  in 
various  books  on  these  subjects.  But  it  is  easily  un- 
derstood by  a  figure.    If  on  the  ball  a  a  ^ 

force  be  impressed  sufficient  to  make  it  ^ — f 
move  with  an  uniform  velocity  to  the  | 

point  b,  in  a  second  of  time  ;  and  if  an-   ^ 

other  force  be  also  impressed  on  the  ball.    Fig.  14. 
which  alone  would  make  it  move  to  the 
point  c,  in  the  same  time  ;  the  ball,  by  means  of  the 
two  forces,  will  describe  the  line  ad,  which  is  a  diago- 
nal of  the  figure,  whose  sides  are  ac  and  ab. 

C.  How  then  is  motion  produced  in  the  direction 
rftheforcel  According fo  the  second  law,  it  ought  to 
be  in  one  case  in  the  direction  ac,  and  in  the  other 
in  that  of  ab,  whereas  it  is  in  that  of  ad. 

F.  Examine  the  figure  a  little  attentively,  carrying 
this  in  your  mind,  that  for  a  body  to  move  in  the 
same  direction,  it  is  7iot  necessary  that  it  should  move 
in  the  same  slrainht  line  ;  but  that  it  is  sufficient  to 
move  either  in  that  line,  or  in  any  one  parallel  to  it. 

C.  I  perceive  then  that  the  ball  when  arrived  at  d, 
has  moved  in  the  direction  ac,  because  bd  is  parallel 
to  ac;  and  also  in  the  direction  ab,  because  cd  is 
parallel  to  it. 


LAWS  OP  MOTION.  41 
F.  And  in  no  other  possible  situation  but  at  the 
point  d  could  this  experiment  be  conformable  to  the 
second  law  of  motion.  When  bodies  move  in  a 
curve,  it  must  be  kept  in  mind  that  there  must  be  a 
continued  action  of  external  force  ;  otherwise,  if  that 
action  were  to  cease  at  any  point,  the  body  would 
continue  its  motion  in  a  straight  line. 


CONVERSATION  XIIT. 

OF  THE  LAWS  OF  MOTION. 

F.  If  you  reflect  a  little  upon  what  we  said  yester- 
day on  the  second  law  of  motion,  you  will  readily  de- 
duce the  following  corollaries.    (Fig.  14.) 

1.  That  if  the  forces  be  equal,  and  act  at  right 
angles  to  one  another,  the  line  described  by  the  ball 
will  be  the  diagonal  of  a  square.  But  in  all  other 
cases,  it  will  be  the  diagonal  of  a  parallelogram  of 
some  kind. 

2.  By  varying  the  angle,  and  the  forces,  you  vary 
the  form  of  your  parallelogram. 

C.  Yes,  papa,  and  I  see  another  consequence,  viz. 
that  the  motions  of  two  forces  acting  conjointly  in  this 
way,  are  not  so  great  as  when  they  act  separately. 

F.  That  is  true,  and  you  are  led  to  the  conclusion, 
I  suppose,  from  the  recollection,  that  in  every  triangle 
any  two  sides  taken  together  are  greater  than  the  re- 
maining side  ;  and  therefore  you  infer,  and  justly  too, 
that  the  motions  which  the  ball  a  must  have  received, 
had  the  forces  been  applied  separately,  would  have 
been  equal  to  ac  and  ab,  or,  which  is  the  same  thing, 
to  ac  and  cd,  the  two  sides  of  the  triangle  adc,  but  by 
their  joint  action,  the  motion  is  only  equal  to  ad,  the 
remaining  side  of  the  triangle. 

Hence  then  you  will  remember,  that  in  the  compo- 
silion,  or  adding  together  of  forces  (as  this  is  called), 
motion  is  always  lost :  and  in  the  resolution  of  any 


42  MECHANICS. 

one  force,  as  ad,  into  two  others,  ac  and  ah,  motion  is 
gained . 

C.  Well,  papa,  but  how  is  it  that  the  heavenly 
bodies,  the  moon  for  instance,  which  is  impelled  by 
two  forces,  performs  her  motion  in  a  circular  curve 
round  the  earth,  and  not  in  a  diagonal  between  the 
du-ection  of  the  projectile  force,  and  that  of  the  attrac- 
tion of  gravity  to  the  earth  ? 

F.  Because,  in  the  case  just  mentioned,  there  was 
but  the  action  of  a  single  impulse  in  each  direction, 
whereas  the  action  of  gravity  on  the  moon  is  con- 
tmual,  and  causes  an  accelerated  motion,  and  hence 
the  line  is  a  curve. 

C.  Supposing  then,  that  a  represent  the  moon,  and 
«c  the  sixteen  feet  through  which  it  would  fall  in  a 
minute  by  the  attraction  of  gravity  towards  the  earth, 
and  ab  represent  the  projectile  force  acting  upon  it  for 
the  same  time.  If  ab  and  ac  acted  as  sinde  impulses, 
the  moon  would  in  that  case  describe  the  diagonal 
ad  ;  but  since  these  forces  are  constantly  acting^  and 
that  of  gravity  is  an  accelerating  force  also,  therefore 
instead  of  the  straight  line  ad,  the  moon  will  be  drawn 
into  the  curve  line  aed.  Do  I  understand  the  matter 
right  ? 

F.  You  do;  and  hence  you  easily  comprehend 
how,  by  good  instruments  and  calculation,  the  attrac- 
tion of  the  earth  upon  the  moon  was  discovered. 

The  ih'ird  law  of  motion,  viz.  that  action  and  re- 
action  are  equal  and  in  contrary  directions,  may  be 
illustrated  by  the  motion  communicated  by  the  per- 
cussion of  elastic  and  non-elastic  bodies. 

E.  \Vhat  are  these,  papa? 

F.  Elastic  bodies  are  those  which  have  a  certain 
spring,  by  which  their  parts  upon  being  pressed  in- 
wards, by  percussion,  return  to  their  former  state  ;  this 
property  is  evident  in  a  ball  of  wool  or  cotton,  or  in 
sponge  compressed.  Non-elastic  bodies  are  those 
which,  when  one  strikes  another,  do  not  rebound,  but 
move  together  after  the  stroke. 


LAWS  OF  MOTION. 
Let  two  equal  ivory  balls  a  and  b  be  sus- 
pended by  threads ;  if  a  be  drawn  a  little 
out  of  the  perpendicular,  and  let  fall  upon  b, 
it  will  lose  its  motion  by  communicating  it 
to  b,  which  will  be  driven  to  a  distance  c,  _^   ^  _ 
equal  to  that  through  which  a  fell;  and  ^1^15, 
hence  it  appears  that  the  re-action  of  b  was 
equal  to  the  action  of  a  upon  it. 

E.  But  do  the  parts  of  the  ivory  balls  yield  by  the 
stroke,  or,  as  you  call  it,  by  the  percussion  1 

F.  They  do  ;  for  if  I  lay  a  little  pamt  on  a,  and  let 
it  couch  b,  it  will  make  but  a  very  small  speck  upon  it ; 
but  if  it  fall  upon  h,  the  speck  will  be  much  larger  ; 
which  proves  that  the  balls  are  elastic,  and  that  a 
little  hollow,  or  dint,  was  made  in  each  by  colli- 
sion. If  now  two  equal  soft  balls  of  clay,  or  glazier  s 
putty,  which  are  non-elastic,  meet  each  other  with 
equal' velocities,  they  would  stop  and  stick  together  at 
the  place  of  their  meeting,  as  their  mutual  actions 
destroy  each  other.  , 

C.  I  have  sometimes  shot  my  white  alley  against 
another  marble  so  plumply,  that  the  marble  has  gone 
off  as  swiftly  as  the  alley  approached  it,  and  that  re- 
mained in  the  place  of  the  marble.  Are  marbles, 
therefore,  as  well  as  ivory,  elastic  ? 

F.  They  are. — If  three  elastic  balls  a, 
b,  c,  be  hung  from  adjoining  centres,  and 
u  be  drawn  a  little  out  of  the  perpendicu- 
lar, and  let  fall  upon  6,  then  will  a  and 
b  become  stationary,  and  q  will  be  driven  ^, 
to  d,  the  distance  through  which  a  fell  O  o  O 
upon  b.  ^^ig- 

If  you  hang  any  number  of  balls,  as 
six,  eight,  &c.  so  as  to  touch  each  other,  and  if  you 
draw  the  outside  one  away  to  a  little  distance,  and 
then  let  it  fall  upon  the  others,  the  ball  upon  the  op- 
posite side  will  be  driven  off,  while  the  rest  remain 
stationary,  so  equally  is  the  action  and  re-action  of 
the  stationary  balls  divided  among  them.  In  the 
same  manner,  if  two  are  drawn  aside  and  suffered 


44 


MECHANICS. 


to  fall  on  the  rest,  tl>e  opposite  two  will  liy  off,  and 
the  others  remain  stationary. 

There  is  one  other  circumstance  depending  upon 
the  action  and  re-action  of  bodies,  and  also  upon 
the  vis  inerti(t  of  matter,  worth  noticing  :  by  some 
authors  you  will  find  it  largely  treated  upon. 

If  I  strike  a  blacksmith's  anvil  with  a  hammer, 
action  and  re-action  being  equal,  the  anvil  strikes  the 
hammer  as  forcibly  as  the  hammer  strikes  the  anvil. 

If  the  anvil  be  large  enough,  I  might  lay  it  on  my 
breast,  and  suffer  you  to  strike  it 'with  a  sledge  hammer 
with  all  your  strength  without  pain  or  risk,  for  the 
vis  ijievtia:  of  the  anvil  resists  the  force  of  the  blow. 
But  if  the  anvil  were  but  a  pound  or  two  in  weight, 
your  blow  would  probably  kill  me. 


CONVERSATION  XIV. 

ON    THE    MECHANICAL  POWERS. 

C.  Will  you  now,  papa,  explain  the  mechanical 
powers  ? 

F,  I  will,  and  I  hope  you  have  not  forgotten  what 
the  momentum  of  a  body  is. 

G.  No,  it  is  the  force  of  a  moving  body,  which 
force  is  to  be  estimated  by  the  weight,  multiplied  into 
its  velocity. 

F.  Then  a  small  body  may  have  an  equal  momen- 
tum with  one  much  larger  ? 

C.  Yes,  provided  the  smaller  body  moves  as  much 
swifter  than  the  larger  one,  as  the  weight  of  the  latter 
is  greater  than  that  of  the  former. 

F,  What  do  you  mean  when  you  say  that  one 
body  moves  swifter,  or  has  a  greater  velocity,  than 
another  ? 

C.  That  it  passes  over  a  greater  space  in  the  same 
time.  Your  watch  will  explain  my  meaning  :  the 
minute-hand  travels  round  the  dial-plate  in  an 
hour,  but  the  hour-hand  takes  twelve  hours  to  perform 
its  course  in,  consequently,  the  velocity  of  the  miiiute- 


MECHANICAL  POWERS.  45 
hand  is  twelve  times  greater  than  that  of  the  hour- 
hand  ;  because,  in  the  same  time,  viz,  tvi^elve  hours, 
it  travels  twelve  times  the  space  that  is  gone  through 
by  the  hour-hand. 

F.  But  this  can  be  only  true  on  the  supposition 
that  the  two  circles  are  equal.  In  my  watch,  the 
minute-hand  is  longer  than  the  other,  and  conse- 
quently, the  circle  described  by  it  is  larger  than  that 
described  by  the  hour-hand. 

C.  I  see  at  once  that  my  reasonmg  holds  good  only 
in  the  case  where  the  hands  are  equal. 

F.  There  is,  however,  a  particular  point  of  the 
longer  hand,  of  which  it  may  be  said,  with  the  strictest 
truth,  that  it  has  exactly  twelve  times  the  velocity  of 
the  extremity  of  the  shorter. 

C.  That  is  the  point  at  which,  if  the  remainder 
were  cut  off,  the  two  hands  would  be  equal.  And, 
in  fact  every  different  point  of  the  hand  describes 
different  spaces  in  the  same  time. 

F.  The  little  pivot  on  which  the  two  hands  seem 
to  move  (for  they  are  really  moved  by  different 
pivots,  one  within  another)  may  be  called  the  centre 
of  motion,  which  is  a  fixed  point ;  and  the  longer  the 
hand  is,  the  greater  is  the  space  described. 

C.  The  extremities  of  the  vanes  of  a  wind-mill, 
when  they  are  going  very  fast,  are  scarcely  distin- 
guishable, though  the  separate  parts,  nearer  the  mill, 
are  easily  discerned  ;  this  is  owing  to  the  velocity  of 
the  extremities  being  so  much  greater  than  that  of 
the  other  parts. 

E.  Did  not  the  swiftness  of  the  round-abouts,  which 
we  saw  at  the  fair,  depend  on  the  same  principle,  viz. 
the  length  of  the  poles  upon  which  the  seats  were  fixed  ! 

F.  Yes  ;  the  greater  the  distance,  at  which  these 
seats  were  placed,  from  the  centre  of  motion,  the 
greater  the  space  which  the  little  boys  and  girls 
travelled  for  their  halfpenny. 

E.  Then  those  in  the  second  row,  had  a  shorter 
ride  for  then  money  than  those  at  the  end  of  the 
pol^s  ? 


4G 


MECHANICS. 


F.  Yes,  shorter  as  to  space,  but  the  same  as  to 
time.  In  the  same  way,  when  you  and  Charles  go 
round  the  gravel -walk  for  half  an  hour's  exercise,  if 
he  run  while  you  walk,  he  will,  perhaps,  have  gone 
six  or  eight  times  round,  in  the  same  time  that  you 
have  been  but  three  or  four  times  ;  now,  as  to  time, 
your  exercise  has  been  equal,  but  he  may  have  passed 
over  double  the  space  in  the  same  time. 

C.  How  does  this  apply  to  the  explanation  of  the 
mechanical  powers? 

F.  You  will  find  the  application  very  easy : — 
without  clear  ideas  of  what  is  meant  by  time  and 
space,  it  were  in  vain  to  expect  you  to  comprehend 
the  principles  of  mechanics. 

There  are  six  mechanical  powers.  The  lever ; 
the  wheel  and  axle  ;  the  pulley ;  the  inclined  plane  ; 
the  wedge  ;  and  the  screw. 

E.  Why  are  they  called  mechanical  powers? 

F.  Because,  by  their  means,  we  are  enabled  me- 
chanically to  raise  weights,  move  heavy  bodies,  and 
overcome  resistances,  which,  without  their  assistance, 
could  not  be  done. 

C.  But  is  there  no  limit  to  the  assistance  gained  by 
these  powers?  for  I  remember  reading  of  Archime- 
des, who  said,  that  with  a  place  for  his  fulcrum  he 
would  move  the  earth  itself. 

F.  Human  power,  with  all  the  assistance  which 
art  can  give,  is  very  soon  limited,  and  upon  this  prin- 
ciple, that  what  ice  gain  in  power,  we  lose  in  time.  That 
is,  if  by  your  own  unassisted  strength,  you  are  able  to 
raise  fifty  pounds  to  a  certain  distance  in  one  minute, 
and  if  by  the  help  of  machinery,  you  wish  to  raise  five 
hundred  pounds  to  the  same  height,  you  will  require 
ten  minutes  to  perform  it  in  ;  thus  you  increase  your 
power  ten-fold,  but  it  is  at  the  expense  of  time.  Or 
in  other  words,  you  are  enabled  to  do  that  with  one 
effort  in  ten  minutes,  which  you  could  have  done  in 
ten  separate  efforts  in  the  same  time. 

E,  The  importance  of  mechanics,  then,  is  not  so 
very  considerable  as  one,  at  first  sight,  would  ima- 


MECHANICAL  POWETIS.  47 
gine  ;  since  there  is  no  real  gain  offeree  acquired  by 
the  mechanical  powers.  . 

F.  Thouoh  there  be  not  any  actual  mcrease  ot 
force  gamed°by  these  powers,  yet  the  advantages  which 
men  derive  from  them  are  inestimable.  It  there  are 
several  small  weights,  manageable  by  human  strength, 
to  be  raised  to  a  certain  height,  it  may  be  full  as  con- 
venient to  elevate  them  one  by  one,  as  to  take  the  ad- 
vantao-e  of  the  mechanical  powers,  in  raismg  them  all 
at  once.  Because,  as  we  have  shewn,  the  same  time 
will  be  necessary  in  both  cases.  But  suppose  you 
have  a  large  block  of  stone  of  a  ton  weight  to  carry 
away,  or  a  weight  still  greater,  what  is  to  be  done  { 

E.  I  did  not  think  of  that,  . 

F.  Bodies  of  this  kind  cannot  be  separated  mto 
parts  proportionable  to  the  human  strength  without 
immense  labour,  nor,  perhaps,  without  rendering  them 
unfit  for  those  purposes  for  which  they  are  to  be  ap- 
plied. Hence  then  you  perceive  the  great  importance 
of  the  mechanical  powers,  by  the  use  of  which,  a  man 
is  able  with  ease  to  manage  a  weight  many  times 
p:reater  than  himself.  , 

CI  have,  indeed,  seen  a  few  men,  by  means  ot 
pulleys,  and  seemingly  with  no  very  great  exertion, 
raise  an  enormous  oak  into  a  timber-carnage,  in  order 
to  convey  it  to  the  dock-yard.        ,         ,     ,      .  . 

F.  A  very  excellent  instance;  for  if  the  tree  had 
been  cut  into  such  pieces  as  could  have  been  ma- 
naged by  the  natural  strength  of  these  men,  it  would 
not  have  been  worth  carrying  to  Deptford  or  Chat- 
ham for  the  purpose  of  ship-building. 

E.  1  acknowledge  my  error     what  is  a  fulcrum, 

^^E.  It  is  a  fixed  point,  or  prop,  round  which  the 
other  parts  of  a  machine  move. 

C.  The  pivot,  upon  which  the  hands  of  your  watch 
move,  is  a  fulcrum  then  ?  i  ^ 

E  It  is  and  you  remember  we  called  it  also  the 
centre  of  motion  the  rivet  of  these  scissars  is  also  a 
fulcrum,  and  also  the  centre  of  motion. 


48 


MECHANICS. 


E.  Is  tliat  a  fixed  point,  or  prop  ? 

F.  Certainly  it  is  a  fixed  point,  as  it  regards  the 
two  parts  of  the  scissars  ;  for  that  always  remains 
m  the  same  position,  while  the  other  parts  move 
about  it.  Take  the  poker  and  stir  the  fire ;  now  that 
part  of  the  bar  on  which  the  poker  rests  is  a  fulcrum, 
for  the  poker  moves  upon  it  as  a  centre.  ' 


CONVERSATION  XV. 

OF  THE  LEVER. 

F.  We  will  now  consider  the  Lever,  which  is 
generally  called  the  first  mechanical  power. 

The  lerer  is  any  inflexible  bar  of  wood,  iron,  &c. 
which  serves  to  raise  weights,  while  it  is  supported  at 
a  point  by  a  prop  or  fulcrum,  on  which,  as  the  centre 
of  motion,  all  the  other  parts  turn,  tt 6  , 
will  represent  a  lever,  and  the  point  c  „  ^.-^1 
the  fulcrum  or  centre  of  motion.  Now 
it  is  evident,  if  the  lever  turn  on  its  \r'' 
centre  of  motion  c,  so  that  b  comes  ^  ^. 
into  the  position  ti;  a  at  the  same  time  §'* 
must  come  into  the  position  e.    If  both  the  arms  of 
the  lever  be  equal,  that  is,  if  ac  is  equal  to  he,  there 
is  no  advantage  gained  by  it,  for  they  pass  over  equal 
spaces  in  the  same  time ;  and,  according  to  the  fun- 
damental principle  already  laid  down,  (p.  46,)  "  as 
advantage  or  power  is  gained,  time  must  be 'lost:" 
therefore,  no  time  being  lost  by  a  lever  of  this  kind, 
there  can  be  no  power  gained. 

^Vhy  then  is  it  called  a  mechanical  power  ? 

F.  Strictly  speaking,  perhaps,  it  ought  not  to  be 
numbered  as  one.  But  it  is  usually  reckoned  among 
them,  having  the  fulcrum  between  the  weight  and  the 
])ow{?i ,  uliich  is  the  distinguishing  property  of  levers 
of  tlie  liisl  kind.  And  when  the  fulcrum  is  exactly 
the  middle  point  between  the  weight  and  power  it  is 
the  common  balance  :  to  which,  if  scales  be  sus- 


MECHANICAL  POWEllS.  49 
ponded  at  a  and  b,  it  is  fitted  for  weighing  all  sorts  of 
(jommodities. 

E.  You  say  it  is  a  lever  of  the  first  kind  j  are  there 
several  sorts  of  levers  1 

F,  I'here  are  three  sorts ;  some  persons  reckon 
four ;  the  fourth,  however,  is  but  a  bended  one  of  thtj 
first  kind.  A  lever  of  the  fir bt  kind  has  the  fulcrum 
betv/een  the  weight  and  power. 


6^ 


4      '  "fl 


Fig.  18.  ^^ig- 
The  second  kind  of  lever  has  the  fulcrum  at  one 
end,  the  power  at  the  other,  and  the  weight  between 
them.  ^ 

Fig.  20.  Fig.  21. 

In  the  third  kind,  the  power  is  between  the  ful- 
crum and  the  weight.  .  .   ,   /-n-     m  X 

Let  us  take  the  lever  of  the  first  kmd,  (Fig.  18.) 
which  if  it  be  moved  into  the  position  cd,  by  turning 
on  its  fulcrum  e,  it  is  evident  that  while  a  has  tra- 
velled over  the  short  space  ac,  b  has  travelled  over 
the  greater  space  bd,  which  spaces  are  to  one  another 
exactly  in  proportion  to  the  length  of  the  arms  ae  and 
be.  If,  now,  you  apply  your  hand  first  to  the  point 
a,  and  afterwards  to  b,  in  order  to  move  the  lever 
into  the  position  cd,  using  the  same  velocity  in  both 
cases,  you  will  find,  that  the  time  spent  in  moving  the 
lever  when  the  hand  is  at  b,  will  be  as  much  greater, 
as  that  spent  when  the  hand  is  at  a,  as  the  arm  be  is 
longer  than  the  arm  ae;  but  then  the  exertion  re- 
quired will,  in  the  same  proportion,  be  less  at  b  than 
at  a. 

C.  The  arm  be  appears  to  be  four  times  the  length 
of  ae. 

I) 


50 


MECHANICS. 


F.  Then  it  is  a  lever  which  gains  power  in  the 
proportion  of  four  to  one.  That  is,  a  single  pound 
weight  applied  to  the  end  of  the  arm  be,  as  at  p,  will 
balance  four  pounds  suspended  at  a,  as  u\ 

C.  1  have  seen  workmen  move  large  pieces  of  tim- 
ber to  very  small  distances,  by  means  of  a  long  bar 
of  wood  or  iron  ;  is  that  a  lever  1 

F,  It  is ;  they  force  one  end  of  the  bar  under  the 
timber,  and  then  place  a  block  of  wood,  stone,  &c. 
beneath,  and  as  near  the  same  end  of  the  lever  as 
possible,  for  a  fulcrum,  applying  their  own  strength 
to  the  other :  and  power  is  gained  in  proportion  as 
the  distance  from  the  fulcrum  to  the  part  where  the 
men  apply  their  strength,  is  greater  than  the  distance 
from  the  fulcrum  to  that  end  undei  the  timber.  Hand- 
spikes are  levers  of  this  kind,  and  by  these  the  hea- 
viest cannon  are  moved,  as  well  as  other  heavy 
bodies. 

C.  It  must  be  very  considerable,  for  IJiave  seen 
two  or  three  men  move  a  tree  in  this  way,  of  several 
tons'  weight,  I  should  think, 

F.  That  is  not  difficult ;  for  supposing  a  lever  to 
gain  the  advantage  of  twenty  to  one,  and  a  man  by 
his  natural  strength  is  able  to  move  but  a  hundred 
weight,  he  will  find  that  by  a  lever  of  this  sort  he  can 
move  twenty  hundred  weight,  or  a  ton ;  but,  for 
single  exertions,  a  strong  man  can  put  forth  a  much 
greater  power  than  that  which  is  sufficient  to  remove 
a  hundred  weight ;  and  levers  are  also  frequently 
used,  the  advantage  gained  by  which  is  still  more 
considerable  than  twenty  to  one. 

C.  I  think  you  said,  the  other  day,  that  the  com- 
mon steelyard  made  use  of  by  the  butcher  is  a  lever] 

F.  I  did ;  the  short  arm  ac  (Fig.  19.)  is,  by  an 
increase  in  size,  made  to  balance  the  longer  one  be, 
and  from  c,  the  centre  of  motion,  the  divisions  must 
commence.  Now  if  he  be  divided  into  as  many  parts 
as  it  will  contain,  each  equal  to  ac,  a  single  weight, 
as  a  pound,  will  serve  for  weighing  any  thing  as 
heavy  as  itself,  or  as  many  times  heavier  as  there  are 


OF  THE  LEVER.  51 
divisions  in  the  arm  c.  If  the  weight  p  be  placed  at 
the  division  1  in  the  arm  he,  it  will  balance  one 
pound  in  the  scale  at  «;  if  it  be  removed  to  3,  5,  or 
7  it  will  balance  3,  6,  or  7  pounds  m  the  scale  ;  tor 
these  divisions  being  respectively  3,  5,  or  7  times  tae 
distance  from  the  centre  of  motion  c,  that  a  is,  it  be- 
comes a  lever,  which  gains  advantage,  in  those  points, 
in  the  proportion  of  3,  5,  and  7.  If,  now,  the  mter- 
vals  between  the  divisions  on  the  longer  arm  be  sub- 
divided into  halves,  quarters,  &c.  any  weight  may  be 
accurately  ascertained,  to  halves,  quarters  of  pounds, 
&c. 

CONVEKSATION  XVI. 

OF   THE  LEVER. 

E.  What  advantage  has  the  steelyard,  which  you 
described  in  our  last  conversation,  over  a  pair  of 
scales  1 

F.  it  may  be  much  m.ore  readily  removed  from 
place  to  place  ;  it  requires  no  apparatus,  and  only  a 
single  weight  for  all  the  purposes  to  which  it  can  be 
applied.  —  Sometimes  the  arms  are  not  of  equal 
weight.  In  that  case  the  weight  p  must  be  moved 
along  the  arm  be,  till  it  exactly  balance  the  other 
arm  without  a  weight,  and  in  that  point  a  notch  must 
be  made,  marking  over  it  a  cypher  0,  from  whence 
the  divisions  must  commence. 

C.  Is  there  not  required  great  accuracy  in  the  ma- 
nufacture of  instruments  of  this  kind  1 

F.  Yes ;  of  such  importance  is  it  to  the  public 
that  there  should  be  no  error  or  fraud  by  means  of 
false  weights,  or  false  balances,  that  it  is  the  business 
of  certain  public  officers  to  examine  at  stated  seasons 
the  weights,  measures,  &c.  of  every  shopkeeper  in 
the  land.  Yet  it  is  to  be  feared  that,  after  ail  pre- 
cautions, much  fraud  is  practised  upon  the  unsus- 
pecting. 

E.  I  one  day  last  summer  bought,  as  I  supposed, 
a  pound  of  cherries  at  the  door ;  but  Charles  think- 


52 


MECHANICS. 


ing  there  was  not  a  pound,  we  tried  them  in  your 
scales,  and  found  but  twelve  ounces,  or  three  quar- 
ters, instead  of  a  pound,  and  yet  the  scale  went  down 
as  if  the  man  had  given  me  full  weight.  How  was 
that  managed  ? 

F,  It  might  be  done  many  ways :  by  short 
weights  ; — or  by  the  scale  in  which  the  fruit  was  put 
being  heavier  than  the  other: — but  fraud  may  be 
practised  with  good  weights  and  even  scales,  by 
making  the  arm  of  the  balance  on  which  the  weights 
hang  shorter  than  the  other,  for  then  a  pound  weight 
v/ill  be  balanced  by  as  much  less  fruit  than  a  pound 
as  that  arm  is  shorter  than  the  other  j  this  was  pro- 
bably the  method  by  v/hich  you  were  cheated. 

E.  By  what  method  could  I  have  discovered  this 
cheat? 

F.  The  scales  when  empty  are  exactly  balanced, 
but  when  loaded,  though  still  in  equilibrio,  the  weights 
are  unequal,  and  the  deceit  is  instantly  discovered  by 
changing  the  weights  to  the  contrary  scales.  I  will 
give  you  a  rule  to  find  the  true  weight  of  any  body 
by  sueh  a  false  balance ;  the  reason  of  the  rule  you 
will  understand  hereafter:  "Jind  the  weights  of  the 
body  by  both  scales,  multiply  them  together,  and  then 
find  the  square  root  of  the  product,  which  is  the  true 
loeight" 

C.  Let  me  see  if  I  understand  the  rule  : — suppose 
a  body  weigh  16  ounces  in  one  scale,  and  in  the  other 
12  ounces  and  a  quarter,  I  multiply  16  by  12  and  a 
quarter,  and  I  get  the  product  196,  the  square  root 
of  which  is  14;  for  14  multiplied  into  itself  gives 
196  ;  therefore  the  true  weight  of  the  body  is  14 
ounces. 

F.  That  is  just  what  I  meant. — To  the  lever  of  the 
first  kind  may  be  referred  many  common  instruments, 
such  as  scissars,  pincers,  snuffers,  &c.  which  are 
made  of  two  levers,  acting  contrary  to  one  another. 

F,  The  rivet  is  the  fulcrum,  or  centre  of  motion, 
the  hand  the  power  used,  and  whatever  is  to  be  cut  is 
the  resistance  to  be  overcome. 


OP  THE  LEVER.  53 

C.  A  poker  stirriog  the  fire  is  also  a  lever,  for  the 
bar  is  the  fulcrum,  the  hand  the  power,  and  the  coals 
the  resistance  to  be  overcome. 

F,  W e  nov^  proceed  to  levers  of  the  second  kind, 
in  which  the  fulcrum  c  (Fig.  20.)  is  at  one  end,  the 
power  p  applied  at  the  other  b,  and  the  weight  to 
be  raised  at  iv,  somewhere  between  the  fulcrum  and 
the  power. 

C.  And  how  is  the  advantage  gained  to  be  esti- 
mated in  this  lever? 

F,  By  looking  at  the  figure,  you  will  find  that 
power  or  advantage  is  gained  in  proportion  as  the  dis- 
tance of  the  power  p  is  greater  than  the  distance  of 
the  weight  iv  from  the  fulcrum. 

C.  Then  if  the  weight  hang  at  one  inch  from  the 
fulcrum,  and  the  power  acts  at  five  inches  from  it, 
the  power  gained  is  five  to  one,  or  one  pound  at  p 
will  balance  five  at  ivl. 

F.  It  will  3  for  you  perceive  that  the  power  passes 
over  five  tirhes  as  great  a  space  as  the  weight,  or 
while  the  point  e  in  the  lever  moves  over  one  inch, 
the  point  h  will  move  over  five  inches. 

E,  What  things  in  common  use  are  to  be  referred 
to  the  lever  of  the  second  kind  ? 

F.  The  most  common  and  useful  of  all  things; 
every  door,  for  instance,  which  turns  on  hinges  is  a 
lever  of  this  sort.  The  hinges  may  be  considered  as 
the  fulcrum,  or  centre  of  motion,  the  whole  door  is 
the  weight  to  be  moved,  and  the  power  is  applied  to 
that  side  on  which  the  lock  is  usually  fixed. 

E.  Now  I  see  the  reason  why  there  is  considerable 
difficulty  in  pushing  open  a  heavy  door,  if  the  hand  is 
applied  to  the  part  next  the  hinges,  although  it  may 
be  opened  with  the  greatest  ease  in  the  usual  method, 

C.  This  sofa,  with  sister  upon  it,  represents  a  lever 
of  the  second  kind. 

F,  Certainly  ;  if  while  she  is  sitting  upon  it,  in  the 
middle,  you  raise  one  end,  while  the  other  remains 
fixed  as  a  prop  or  fulcrum.  To  this  kind  of  lever 
may  be  also  reduced  nut-crackers  j  oars ;  rudders  of 


51 


MECHANICS. 


ships  ;  those  cutting  knives  which  have  one  end  fixed 
in  a  block,  such  as  are  used  for  cutting  chaff,  drugs, 
wood  for  pattens,  &c. 

E.  I  do  not  see  how  oars  and  rudders  are  levers  of 
this  sort. 

jp.  The  boat  is  the  weight  to  be  moved,  the  w^ater 
is  the  fulcrum,  and  the  waterman  at  the  handle  the 
power.  The  masts  of  ships  are  also  levers  of  the 
second  kind,  for  the  bottom  of  the  vessel  is  the  ful- 
crum, the  ship  the  weight,  and  the  wind  acting 
against  the  sail  is  the  moving  power. 

The  knowledge  of  this  principle  may  be  useful  in 
many  situations  and  circumstances  of  life : — if  two 
men  unequal  in  strength  have  a  heavy  burden  to 
carry  on  a  pole  between  them,  the  ability  of  each 
may  be  consulted,  by  placing  the  burden  as  much 
nearer  to  the  stronger  man,  as  his  strength  is  greater 
than  that  of  his  partner. 

E.  Which  would  you  call  the  prop  in  this  case  1 

F,  The  stronger  man,  for  the  weight  is  nearest  to 
liim  ;  and  then  the  weaker  must  be  considered  as  the 
power.  Again,  two  horses  may  be  so  yoked  to  a 
carriage  that  each  shall  draw  a  part  proportional  to 
his  strength,  by  dividing  the  beam  in  such  a  manner 
that  the  point  of  traction,  or  drawing,  may  be  as 
much  nearer  to  the  stronger  horse  than  to  the  weaker, 
as  the  strength  of  the  former  exceeds  that  of  the 
latter. 

We  will  now  describe  the  third  kind  of  lever.  In 
this  the  prop  or  fulcrum  e  (Fig.  21.)  is  at  one  end, 
the  weight  w  at  the  other,  and  the  power  p  is  applied 
at  b,  somewhere  between  the  prop  and  weight. 

C.  In  this  case,  the  weight  being  farther  from  the 
centre  of  motion  than  the  power,  must  pass  through 
more  space  than  it. 

F,  And  what  is  the  consequence  of  that  ? 

C.  That  the  power  must  be  greater  than  the  weight, 
and  as  much  greater  as  the  distance  of  the  weight 
from  the  prop  exceeds  the  distance  of  the  power  from 
it,  that  is,  to  balance  a  weight  of  three  pounds  at  a, 


OF  THE  WHEEL  AND  AXIS.  55 
tlieie  will  require  tlie  exertion  of  a  power  p,  acting  at 

l^tn'ef  tbent;er  of  this  kind  is  a  disad.an- 
taoVto  the  moving  power,  it  is  but  seldom  used,  and 
oTy  n  cases  of  nec'^.ssity  ;  such  as  m  tna  of  a  ad- 
der! which  being  fixed  a^       -d  ^^I.:";;^ 
S  tfot^W^enlicufarluUion  .  But  the  most 
IZortan   application  of  this  third  kind  of  lever,  is 
Tan"  esHn  the  structure  of  the  limbs  of  animals,  par- 
Tuto  V  in  those  of  man;  to  take  the  f^^^^^^X 
stoce    when  we  lift  a  weight  by  the  hand,  it  is 
effected  by  means  of  muscles  coming  from  the 
shoulder-blade,  and  terminating  about  one-tenth  as 
far  below  the  elbow  as  the  hand  is:  now  the  elbow 
being  the  centre  of  motion  round  which  the  lower 
mrt  of  the  arm  turns,  according  to  the  prmciple  just 
laTd  down,  the  muscles  must  exert  a  force  ten  times 
i  treat^  the  weight  that  is  raised.    At  first  view 
this  may  appear  a  disadvantage,  but  what  is  lost  m 
powe"  iJ  gained  in  velocity,  and  thus  the  human 
^gure  is  better  adapted  to  the  various  functions  it  has 
to  perform. 


CONVERSATION  XVIL 

OF  THE  WHEEL  AI^D  AXIS. 

F  Well,  Emma,  do  you  understand  the  principle 
of  the  lever,  which  we  discussed  so  much  at  large 

^'^E^'l^he' lever  gains  advantage  in  proportion  to  the 
space  passed  through  by  the  acting  power  ;  that  is  it 
the  wetht  to  be  raled  be  at  the  distance  of  one  inch 
from  he  fulcrum,  and  the  power  is  applied  nme 
nches  distant  from  it,  then  it  is  a  lever  which  gams 
advantage  as.  nine  to  one,  because  the  space  passed 
through  by  the  po^i-.r  is  nine  times  greater  than  that 
passed  through  by  the  weight ;  and,  therefore,  what 


50  MECHANICS. 

is  lost  in  time,  by  passing  through  a  greater  space»  is 
gamed  m  power. 

F.  You  recollect  also  what  the  different  kinds  of 
levers  are,  I  hope  ? 

E.  I  shall  never  see  the  fire  stirred  without  think- 
mg  of  a  simple  lever  of  the  first  kind ;  my  scissars 
wiil  frequently  remind  me  of  a  com.bination  of  two  - 
levers  of  the  same  sort.  The  opening  and  shuttino- 
of  the  door  will  prevent  me  from  forgetting  the  na° 
ture  of  the  lever  of  the  second  kind ;  and  I  am  sure 
that  I  shall  never  see  a  workman  raise  a  ladder 
against  a  house  without  recollecting  the  third  sort  of 
lever.  Besides,  I  believe  a  pair  of  tongs  is  a  lever  of 
this  kind  ? 

i^.  You  are  right ;  for  the  fulcrum  is  at  the  joint, 
and  the  power  is  applied  between  that  and  the  parts 
used  m  taking  up  coals,  &c.~Can  you,  Charles,  tell 
us  how  the  principle  of  momentum  applies  to  the 
lever  ? 

C.  The  momentum  of  a  body  is  estimated  by  its 
weight  multiplied  into  its  velocity;  and  the  velocity 
must  be  calculated  by  the  space  passed  through  in  a 
given  time.  Now,  if  I  examine  the  lever  (iio-.  18. 
20.)  and  consider  it  as  an  inflexible  bar  turning  on  a 
centre  of  motion,  it  is  evident  that  the  same  time  is 
used  for  the  motion  both  of  the  weight  and  the  power 
but  the  spaces  passed  over  are  very  different ;  that 
which  the  power  passes  through  being  as  much 
greater  than  that  passed  by  the  weight,  as  the  leno-th 
of  the  distance  of  the  power  from  the  prop  is  crrealer 
tnan  the  distance  of  the  weight  from  the  prop'';  and 
the  velocities  being  as  the  spaces  passed  in  the  same 
time,  must  be  greater  in  the  same  proportion.  Con- 
sequently, the  velocity  of  p,  the  power,  multiplied 
into  its  weight,  wall  be  equal  to  the  smaller  velocity 
of  10,  multiplied  into  its  weight,  and  thus  their  mo- 
"'T'^^.n^.^'S-  equal,  they  will  balance  one  anotlier. 

^.  Ihis  applies  to  the  first  and  second  kind  of 
lever  ;  what  do  you  say  to  the  third  ? 

C.  In  the  third,  the  velocity  of  the  power;)  (Ih 


OF  THE  WHEEL  AND  AXIS.  57 
21.)  being  less  than  that  of  the  weight  it  is  evi- 
dent, in  order  that  their  momenta  may  be  equal,  that 
the  weight  acting  at  p  must  be  as  much  greater  than 
that  of  lu  as  ae  is  less  than  be,  and  then  they  will  be 
in  equiUbrio. 

F.  The  second  mechanical  power  is  the  Wheel  and 
Axis,  which  gains  power  in  proportion  as  the  circum-. 
ference  of  the  wheel  is  greater  than  that  of  the  axis  • 
this  machine  may  be  referred  to 
the  principle  of  the  lever,  ah 
is  the  wheel,  cd  its  axis ;  and  if 
the  circumference  of  the  wheel 
be  eight  times  as  great  as  that 
of  the  axis,  then  a  single  pound, 
JO,  will  balance  a  weight,  w,  of 
eight  pounds. 

C.  Is  it  by  an  instrument  of  this  kind  that  water  is 
drawn  from  those  deep  wells  so  common  in  many 
parts  of  the  country  1 

F,  It  is ;  but  as  in  most  cases  of  this  kind  only  a 
single  bucket  is  raised  at  once,  there  requires  but 
little  power  in  the  operation,  and  therefore,  instead 
of  a  large  wheel,  as  ab,  an  iron  handle  fixed  at  c  is 
made  use  of,  which,  you  know,  by  its  circular  mo- 
tion, answers  the  purpose  of  a  wheel. 

C.  I  once  raised  some  water  by  a  machine  of  this 
kind,  and  I  found  that  as  the  bucket  ascended  nearer 
the  top  the  difficulty  increased. 

F.  That  must  always  be  the  case,  where  the  wells 
are  so  deep  as  to  cause,  in  the  ascent,  the  rope  to  coil 
more  than  once  the  length  of  the  axis,  because  the 
advantage  gained  is  in  proportion  as  the  circumfer- 
ence of  the  wheel  is  greater  than  that  of  the  axis ; 
so  that  if  the  circumference  of  the  wheel  be  12  times 
greater  than  that  of  the  axis,  one  pound  applied  at  the 
former  will  balance  twelve  hanging  at  the  latter ;  but 
by  the  coiling  of  the  rope  round  the  axis,  the  differ* 
ence  between  the  circumference  of  the  wheel  and  that 
of  the  axis  continually  diminishes ;  consequently  the 
advantage  gained  is  lei-^s  every  time  a  new  coil  of 
D2 


5B  MECHANICS. 

rope  is  wound  on  the  whole  length  of  the  axis  :  tliis 

explains  why  the  difficulty  of  drawing  the  v/ater,  or 

any  other  weight,  increases  as  it  ascends  nearer  the 

top. 

C.  Then  by  diminishing  the  axis,  or  by  increasing 
the  length  of  the  handle,  advantage  is  gained  1 

F.  Y es,  by  either  of  these  methods  you  may  gain 
power ;  but  it  is  very  evident  that  the  axis  cannot  be 
diminished  beyond  a  certain  limit,  without  rendering 
it  too  weak  to  sustain  the  weight ;  nor  can  the  handle 
be  managed,  if  it  be  constructed  on  a  scale  much 
larger  than  what  is  commonly  used. 

C.  We  must,  then,  have  recourse  to  the  v;heel 
with  spikes  standing  out  of  it,  at  certain  distances 
from  each  other,  to  serve  as  levers. 

F,  You  may  by  this  means  increase  your  power 
according  to  your  wish,  but  it  must  be  at  the  expense 
of  time,  for  you  know  that  a  simple  handle  may  be 
turned  several  times,  while  you  are  pulling  the  wheel 
round  once. — To  the  principle  of  the  ivJieel  and  axis 
may  be  referred  the  capstan,  windlass,  and  all  those 
numerous  kinds  of  cranes,  which  are  to  be  seen  at 
the  different  wharfs  on  the  banks  of  the  Thames. 

C,  I  have  seen  a  crane,  which  consists  of  a  wheel 
large  enough  for  a  man  to  vvalk  in. 

F.  In  this  the  weight  of  the  man,  or  men  (for 
there  are  sometimes  two  or  three),  is  the  moving 
power ;  for,  as  the  man  steps  forwards,  the  part  upon 
which  he  treads  becomes  the  heaviest,  and  conse- 
quently descends  till  it  be  the  lowest.  On  the  same 
principle,  you  may  see  at  the  door  of  many  bird-cage 
makers,  a  bird,  by  its  weight,  give  a  wicker  cage  a 
circular  motion  ;  now,  if  there  were  a  small  weight 
suspended  to  the  axis  of  the  cage,  the  bird  by  its  mo- 
tion would  draw  it  up,  for,  as  it  hops  from  the  bottom 
bar  to  the  next,  its  momentum  causes  that  to  descend; 
and  thus  the  operation  is  performed,  both  with  regard 
to  the  cage,  and  to  those  large  cranes  which  you 
i  ave  seen. 

E.  Is  there  no  danger  if  the  man  happens  to  slip  1 


OF  THE  WHEEL  AND  AXIS.  59 
F.  Tf  the  weight  be  very  great,  a  slip  with  the  foot 
may  be  attended  with  very  dangerous  consequences. 
To  prevent  which,  there  is  generally  fixed  at  one  end 
of  the  axis  a  little  wheel,/,  (Fig.  22.)  called  a 
racket-wheel,  with  a  catch,  e,  to  fall  into  its  teeth ; 
this  will,  at  any  time,  support  the  weight  in  case_  of 
an  accident.  Sometimes,  instead  of  men  walking 
within  the  great  wheel,  cogs  are  set  round  it  on  the 
outside,  and  a  small  trundle-wheel  made  to  work  in 
the  cogs,  and  to  be  turned  by  a  winch. 

C.  Are  there  not  other  sorts  of  cranes,  in  which  all 
danger  is  avoided  ^. 

F.  The  crane  is  a  machine  of  such  importance  to 
the  commercial  interests  of  this  country,  that  new  in- 
ventions of  it  are  continually  offered  to  the  public  : 
I  will,  when  we  go  to  the  library,  shew  you  in  the 
10th  vol.  of  the  Transactions  of  the  Society  for  the 
Encouragement  of  Arts  and  Sciences,  an  engraving 
of  a  safe,  and,  I  believe,  truly  excellent  crane.  It 
was  invented  by  a  friend  of  mine,  Mr.  James  White, 
who  possessed  a  most  extraordinary  genius  for  me 
chanics. 

C.  You  said  that  this  mechanical  power  might  be 
considered  as  a  lever  of  the  first  kind. 

F,  1  did ;  and  if  you  conceive  the  wheel  and  axis 
to  be  cut  through  the  middle  in  the  di- 
rection ab,  fgb  will  represent  a  section 
of  it.  ab  is  a  lever,  whose  centre  of  mo-  /'( 
lion  is  c;  the  weight  w,  sustained  by  the 
rope  aw,  is  applied  at  the  distance  ca, 
the  radius  of  the  axis  ;  and  the  power  p, 
acting  in  the  direction  bp,  is  applied  at  -p.  23 
the  distance  cb,  the  radius  of  the  wheel ;  &•  * 
therefore,  according  to  the  principle  of  the  lever,  the 
power  will  balance  the  v/eight  when  it  is  as  much 
less  than  the  weight  as  the  distance  cb  is  greater  than 
the  distance  of  the  weight  ac. 


00 


MECHANICS. 


CONVERSATION  XVIII. 

OF  THE  PULLEY. 

-F.  The  third  mechanical  power,  the  Pulley,  may 
be  likewise  explained  on  the  principle  of  the  lever. 

The  Ime  ab  may  be  conceived  to  be  a   

lever,  whose  arms  uc  and  be  are  equal,  '^^^'^^ 
and  c  the  fulcrum,  or  centre  of  motion.  H 
If  now  two  equal  weights,  w  and  p,  be    aPS^  'f) 
hung  on  the  cord  passing  over  the  pulley, 
they  will  balance  one  another,  and  the 
fulcrum  will  sustain  both.  O'^'K) 

C.  Does  this  pulley,  then,  like  the  com-  Fig.  24 
mon  balance,  give  no  advantage  ?  ' 

F.  From  the  single  fixed  pulley  no  mechanical 
advantage  is  derived :  it  is,  nevertheless,  of  great  im- 
portance in  changing  the  direction  of  a  power,  and  is 
very  much  used  in  buildings  for  drawing  up  small 
weights,  It  being  much  easier  for  a  man  to  raise  such 
burthens  by  means  of  a  single  pulley,  than  to  carry 
them  up  a  long  ladder. 

E,  Why  is  it  called  a  mechanical  power  ? 

F ,  Though  a  single  fixed  pulley  gives  no  advan- 
tage, yet  when  it  is  not  fixed,  or  v/hen  two  or  more  are 
combined  into  what  is  called  a  system  of  pulleys, 
they  then  possess  all  the  properties  of  the  other  me- 
chanical powers.   Thus  in  cdb  c  is  the  ful-  ^  

crum;  therefore  a  power  p  acting  at  h,  vVl 
will  sustain  a  double  weight  w,  acting  at    \f  ^ 
a,  for  be  is  double  the  distance  of  ac  from 
the  fulcrum,  | 

Again,  it  is  evident,  in  the  present  case, 
that  the  whole  weight  is  sustained  by  the 
cord  edp,  and  whatever  sustains  half  the  'O 
cord,  sustains  also  half  the  weight ;  but    Fig.  25. 
one  half  is  sustained  by  the  fixed  hook 
e,  consequently  the  powei  at  p  has  only  the  other 


OF  THE  PULLEY.  01 
half  to  sustain,  or,  iu  other  words,  any  given  power 
at  p  will  keep  in  equilibrio  a  double  weight  at  w. 

C.  Is  the  velocity  of  p  double  that  of  ly'? 

F  Undoubtedly if  you  compare  the  space  passed 
throuoh  by  the  hand  at  p  with  that  passed  by  w  you 
will  find  that  the  former  is  just  double  of  the  latter, 
and  therefore  the  momenta  of  the  power  and  weight, 
as  in  the  lever,  are  equal. 

C  I  think  J  see  the  reason  of  this;  tor  it  the 
wekht  be  raised  an  inch,  or  a  foot,  both  sides  of  the 
cord  must  also  be  raised  an  inch,  or  foot,  but  this 
cannot  happen  without  that  part  of  the  cord  at  p 
passing  through  two  inches  or  two  feet  oi  space. 

F.  You  will  now  easily  infer,  from  what 
has  been  already  shewn  of  the  single  move- 
able  pulley,  that  in  a  system  of  pulleys  the 
power  gained  must  be  estimated  by  dou- 
bling the  number  of  pulleys  in  the  lower 
or  moveable  block.  So  that  when  the 
fixed  block  a  contains  two  pulleys  which 
only  turn  on  their  axes,  and  the  lower 
block  b  contains  also  two  pulleys,  which 
not  only  turn  on  their  axes,  but  also  rise 
with  the  weight,  the  advantage  is  as  four ; 
that  is,  a  single  pound  at  p  will  sustain  four 

C.  In  the  present  instance,  also,  1  per-  lig.  ^t), 
ceive,  that  by  raising  w  an  inch,  there  are 
four  ropes  shortened  each  an  inch,  and  therefore  the 
hand  must  have  passed  through  four  inches  of  space 
in  raising  the  weight  a  single  inch;  which  establishes 
the  .maxim  that  what  is  gained  in  power  is  lost  m 
space.  But,  papa,  you  have  only  talked  ot  the 
power  balancing  or  sustaining  the  weight ;  something 
more  must,  I  suppose,  be  added  to  raise  it. 

F.  There  must^;  considerable  allowance  must  like- 
wise'be  made  for  the  friction  of  the  cords,  and  of  the 
pivots,  or  axes,  on  which  the  pulleys  turn.  In  the 
mechanical  powers,  in  general,  one-third  ot  power 
must  be  added  for  the  loss  sustained  by  tnction,  and 


02 


MECHANICS. 


for  the  imperfect  manner  in  which  machines  are  com- 
monly constructed.  Thus,  if  by  theory  you  gain  a 
power  of  600,  in  practice  you  must  reckon  only  upon 
400.  In  those  pulleys  which  we  have  been  describ- 
ing, writers  have  taken  notice  of  three  things,  which 
take  much  from  the  general  advantage  and  conve- 
nience of  pulleys  as  a  mechanical  power.  The  Jirst 
is,  that  the  diameters  of  the  axes  bear  a  great  propor- 
tion to  their  own  diameters.  The  secojid  is,  that  in 
working  they  are  apt  to  rub  against  one  another,  or 
against  the  side  of  the  block.  And  the  third  disad- 
vantage is  the  stiffness  of  the  rope  that  goes  over  and 
under  them. 

The  two  first  objections  have  been,  in  a 
great  degree,  removed  by  the  concentric 
pulley,  invented  by  Mr.  Jam.es  White  : 
6  is  a  solid  block  of  brass,  in  which 
grooves  are  cut,  in  the  proportion  of  1, 
3,  5,  7,  9,  &c.  and  a  is  another  block  of 
the  same  kind,  whose  grooves  are  in  the 
proportion  of  2,  4,  6,  8, 10,  &c.  and  round 
these  grooves  a  cord  is  passed,  by  which 
means  they  answer  the  purpose  of  so 
many  distinct  pulleys,  every  point  of 
which  moving  with  the  velocity  of  the 
string  in  contact  with  it,  the  whole  friction  p. 
is  removed  to  the  tvv^o  centres  of  motion 
of  the  blocks  a  and  b  ;  besides,  it  is  of  no  small 
advantage,  that,  the  pulleys  being  all  of  one  piece, 
there  is  no  rubbing  one  against  the  other. 

E.  Do  you  calculate  the  power  gained  by  this  pul- 
ley in  the  same  method  as  with  the  common  pulleys  ? 

-F.  Yes,  for  pulleys  of  every  kind  the  rule  is  gene- 
ral ;  the  advantage  gained  is  found  by  doubling  the 
number  of  the  pulleys  in  the  lower  block  :  in  that 
before  you  there  are  six  grooves,  which  answer  to  as 
many  distinct  pulleys,  and  consequently  the  power 
gained  is  twelve,  or  one  pound  at  p  will  balance 
twelve  pounds  at  w. 


OF  THE  INCLINED  PLANE. 


63 


CONVERSATION  XIX. 

OF  THE  INCLINED  PLANE. 

F.  We  may  now  describe  the  inclined  plane, 
which  is  the  fourth  mechanical  power. 

C.  You  will  not  be  able,  I  think,  to  reduce  this 
also  to  the  principle  of  the  lever. 

F,  No,  it  is  a  distinct  principle,  and  some  writers 
on  these  subjects  reduce  at  once  the  six  mechanical 
powers  to  two,  viz.  the  lever  and  the  inclined  plane, 

E.  How  do  you  estimate  the  advantage  gained  by 
this  mechanical  power  1 

F.  The  method  is  very  easy,  for  just  as  much  as 
the  length  of  the  plane  exceeds  its  perpendicular 
height,  so  much  is  the  advantage 
gained.  Suppose  ah  is  a  plane  stand- 
ing on  the  table,  and  cd  another 
plane  inclined  to  it ;  if  the  length 
cd  be  three  times  greater  than  the  ^  ^ 
perpendicular  height,  then  the  cy-        pig.  28. 
Under  e  will  be  supported  upon  the 

plane  cd  by  a  weight  equal  to  the  third  part  of  its 
own  weight. 

E.  Could  I  then  dra  w  up  a  weight  on  such  a  plane 
with  a  third  part  of  the  strength  that  I  must  exert  in 
lifting  it  up  at  the  end'? 

F,  Ceitaialy  you  might;  allowance,  however, 
must  be  made  for  overcoming  the  friction  ;  but  then 
you  perceive,  as  in  the  other  mechanical  powers,  that 
you  will  have  three  times  the  space  to  pass  over,  or 
that  as  you  gain  power  you  will  lose  time. 

C.  Now  I  understand  the  reason  why  sometimes 
there  are  two  or  three  strong  planks  laid  from  the 
street  to  the  ground-floor  warehouses,  making  there- 
with an  inclined  plane,  on  which  heavy  packages  are 
raised  or  lowered. 

E.  The  inclined  plane  is  chiefly  used  for  raising 
heavy  weights  io  small  heights,  for  in  warehouses  situ- 


Gl 


MECHANICS. 


ated  in  the  upper  part  of  buildings,  cranes  and  pulleys 
are  better  adapted  for  the  purpose. 

C.  I  have  sometimes,  papa,  amused  myself  by  ob- 
serving the  difference  of  time  which  one  marble  has 
taken  to  roll  down  a  smooth  board,  and  another 
which  has  fallen  by  its  own  gravity  without  any  sup- 
port. 

-F.  And  if  it  were  a  long  plank,  and  you  took  care 
to  let  both  marbles  drop  from  the  hand  at  the  same 
instant,  I  dare  say  you  found  the  difference  very 
evident. 

C.  I  did,  and  now  you  have  enabled  me  to  account 
for  it  very  satisfactorily,  by  shewing  me  that  as  much 
more  time  is  spent  in  raising  a  body  along  an  inclined 
plane,  than  in  lifting  it  up  at  the  end,  as  that  plane  is 
longer  than  its  perpendicular  height.  For  I  take  it 
for  granted  that  the  rule  holds  in  the  descent  as  well 
as  in  the  ascent. 

F.  If  you  have  any  doubt  remaining,  a  few  words 
will  make  every  thing  clear.  Suppose  your  marbles 
placed  on  a  plane,  perfectly  horizontal,  as  on  tliis 
table,  they  will  remain  at  rest  wherever  they  are 
placed :  now  if  you  elevated  the  plane  in  such  a 
manner  that  its  height  should  be  equal  to  half  the 
length  of  the  plane,  it  is  evident  from  what  has  been 
shewn  before,  that  the  marbles  would  require  a  force 
equal  to  half  their  weight  to  sustain  them  in  any  par- 
ticular position  :  suppose  then  the  plane  perpendicu- 
lar to  the  table,  the  marbles  will  descend  with  their 
whole  weight,  for  now  the  plane  contributes  in  no 
respect  to  support  them,  consequently  they  would  re- 
quire a  power  equal  to  their  whole  weight  to  keep 
tliem  from  descending. 

C.  And  the  swiftness  with  which  a  body  falls  is  to 
be  estimated  by  the  force  with  which  it  is  acted  upon  ? 

F.  Certainly ;  for  you  are  now  sufficiently  ac- 
quainted with  philosophy  to  know  that  the  efiect  must 
be  estimated  from  the  cause.  Suppose  an  inclined 
plane  is  thirty-two  feet  long,  and  its  perpendicular 
height  is  sixteen  feet,  what  time  will  a  marble  take  in 


OF  THE  WEDGE. 


65 


falling  down  the  plane,  and  also  in  descending  from 
the  to^p  to  the  earth  by  the  force  of  gravity '? 

C  By  the  attraction  of  gravitation,  a  body  talis 
sixteen  feet  in  a  second ;  therefore  the  marble  will  be 
one  second  in  falling  perpendicularly  to  the  ground ; 
and  as  the  length  of  the  plane  is  double  its  height, 
the  marble  must  take  two  seconds  to  roll  t^own  it. 

F.  I  will  try  you  with  another  example.  If  there 
be  a' plane  64  feet  perpendicular  height,  and  3  times 
64,  or  192  feet  long,  tell  me  what  time  a  marble  will 
take  in  fallino-  to  the  earth  by  the  attraction  of  gravity, 
and  how  long  it  will  be  in  descending  down  the 

^^^C.^  By  the  attraction  of  gravity  it  will  fall  in  two 
seconds;  because,  by  multiplying  the  sixteen  feet 
which  it  falls  in  the  first  second,  by  the  square  of  tvvo 
seconds  (the  time),  or  four,  I  get  sixty-four,  the 
heii^ht  of  the  plane.  But  the  plane  being  three  times 
as  fong  as  it  is  perpendicularly  high,  it  must  be  three 
times  as  many  seconds  in  rolling  down  the  plane,  as 
it  was  in  descending  freely  by  the  force  of  gravity, 
that  is,  six  seconds. 

E.  Pray,  papa,  what  common  instruments  are  to 
be  referred  to  this  mechanical  power,  in  the  same 
way  as  scissars,  pincers,  &c.  are  referred  to  the  lever? 

F.  Chisels,  hatchets,  and  whatever  other  sharp  in- 
struments which  are  chamfered,  or  sloped  down  to  an 
edge  on  one  side  only,  may  be  referred  to  the  princi- 
ple of  the  inclined  plane. 


F.  The  next  mechanical  power  is  the  wedge,  which 
is  made  up  of  the  two  inclined  planes  ^ 
ffe/  and  cef  joined  together  at  their  bases 


is  applied,  and  df  and  cf  are  the  length 
of  its  sides ,  now  there  will  be  an  equili-    -^.^^  "29, 
brium  between  the  power  impelling  the 


CONVERSATION  XX. 


OF  THE  WEDGE. 


hrfg:  dc  is  the  whole  thickness  of  the 
wedge  at  its  back  abed,  where  the  power 


G(5  MECHANICS, 
wedge  downward,  and  the  resistance  of  the  wood,  or 
other^  substance  acting  against  its  sides,  when  the 
thickness  do  of  the  wedge  is  to  the  length  of  the  two 
sides,  or,  which  is  the  same  thing,  when  half  the 
thickness  de  o(  the  wedge  at  its  back  is  to  the  length 
of  df  one  of  its  sides,  as  the  power  is  to  the  resistance. 

C.  This  is  the  principle  of  the  inclined  plane. 

F.  It  is,  and  notwithstanding  ail  the  disputes  which 
the  methods  of  calculating  the  advantage  gained  by 
the  wedge  have  occasioned,  I  see  no  reason  to  depart 
from  the  opinion  of  those  who  consider  the  wedge  as 
a  double  inclined  plane. 

E.  I  have  seen  people  cleaving  wood  with  wedges, 
but  they  seem  to  have  no  effect,  unless  great  force 
and  great  velocity  are  also  used. 

F.  ]Mo,  the  power  of  the  attraction  of  cohesion,  by 
which  the  parts  of  v/ood  stick  together,  is  so  great  as 
to  require  a  considerable  momentum  to  separate  them. 
Did  you  observe  nothing  else  in  the  operation  worthy 
of  your  attention  ? 

C.  Yes,  I  also  took  notice  that  the  wood  generally 
split  a  little  below  the  place  to  which  the  wedge 
reached.  ° 

F,  This  happens  in  cleaving  most  kinds  of  wood, 
and  then  the  advantage  gained  by  this  mechanical 
power,  must  be  in  proportion  as  the  length  of  the 
sides  of  the  cleft  in  the  wood  is  greater  than  the 
length  of  the  whole  back  of  the  wedge.  There  are 
other  varieties  in  the  action  of  the  wedge,  but  at 
present  it  is  not  necessary  to  refer  to  them. 

E.  Since  you  said  chat  all  instruments  which  sloped 
of!  to  an  edge  on  one  side  only,  were  to  be  explained 
by  the  principle  of  the  inclined  plane ;  so,  I  suppose, 
that  those  which  decline  to  an  edge  on  both 'sides, 
must  be  referred  to  the  principle  of  the  wedge. 

F.  They  must,  which  is  the  case  with  many 
chisels,  and  almost  all  sort?  of  axes,  &c. 

C.  Is  the  wedge  much  used  as  a  mechanical 
power  ] 

F.  It  is  of  great  importance  in  a  vast  variety  of 
cases,  in  which  the  other  mechanical  powers  are  of 


OF  THE  SCEEW.  07 
110  avail ;  and  this  arises  from  the  momenturn  of  the 
blow,  which  is  greater  beyond  comparison  than  the 
application  of  any  dead  weight  or  pressure,  such  as  is 
employed  in  the  other  mechanical  powers.  Hence  it 
is  used  in  splitting  wood,  rocks,  .^J^f ,  7,^^^^^ 
largest  ship  may  be  raised  to  a  small  height  by  duving 
a  wed^e  below  it.  It  is  also  used  for  raising  up  the 
.  beam  of  a  house  when  the  floor  gives  way,  by  reascm 
of  too  great  a  burden  bemg  laid  upon  it.  It  is  usual 
also  in  separating  large  mill-stones  from  the  silicious 
Id-roclJin  som^e  par\s  of  Derbyshire  to  bore  hor, 
cental  holes  under  them  m  a  circle  and  fi  l  ^^^^se  w  h 
peers  or  wedges  made  of  dry  wood,  which  gradually 
swell  by  the  moisture  of  the  earth,  and  m  a  day  or 
two  lift  up  the  mill-stone  without  breakmg  it. 


CONVERSATION  XXI. 

OF  THE  SCREW. 

F  Let  us  now  examine  the  properties  of  the 
sixth  and  last  mechanical  power,  the  screw  ;  which, 
however,  cannot  be  called 
a  simple  mechanical  power, 
since  it  is  never  used  with- 
out the  assistance  of  a  lever 
or  winch  ;  by  which  it  be- 
comes a  compound  engine, 
of  great  power  in  pressing 
bodies  togetber,  or  in  rais- 
in a-  great  weights,  cib  is 
the  representation  of  one, 

together  with  the  lever  h.  ,     •  , 

F  You  said  just  now,  papa,  that  all  the  mechanical 
powers  were  reducible  either  to  the  lever  or  inclined 
p]nne  :  how  can  the  screw  be  referred  to  either 

F.  The  screw  is  composed  of  two  parts,  one  ot 
which  ab  is  called  the  screw,  and  consists  of  a  spiral 
protuberance,  called  the  thread,  which  may  be  sup- 
posed to  be  wrapt  round  a  cylinder  j  the  other  part,  g, 


68 


ME€HANIC8. 


called  the  nut,  is  perforated  to  the  dimensions  of  the 
cylinder ;  and  in  the  internal  cavity  is  also  a  spiral 
groove  adapted  to  receive  the  thread.  Now  if  you  cut 
a  slip  of  vi^riting-paper  in  the  form  of  an  inclined 
plane  cde,  and  then  wrap  it  round  a  cylinder  of  wood, 
you  will  find  that  it  makes  a  spiral  answering  to  the 
spiral  part  of  the  screw ;  moreover,  if  you  consider 
the  ascent  of  the  screws,  it  will  be  evident  that  it  is 
precisely  the  ascent  of  an-  inclined  plane. 

C.  By  what  means  do  you  calculate  the  advantage 
gamed  by  the  screw  ? 

F.  There  are,  at  first  sight,  evidently  two  things  to 
be  taken  into  consideration  ;  the  first  is  the  distance 
between  the  threads  of  the  screw ; — and  the  second  is 
the  length  of  the  lever. 

C.  Now  I  comprehend  pretty  clearly  how  it  is  an 
mclmed  plane,  and  that  its  ascent  is  more  or  less 
easy  as  the  threads  of  the  spiral  are  nearer  or  farther 
distant  from  each  other. 

F,  Well,  then,  let  me  examine  by  a  question 
whether  your  conceptions  ))e  accurate ;  suppose  two 
screws,  the  circumferences  of  whose  cylinders  are 
equal  to  one  another  ;  but  in  one,  the  distance  of  the 
threads  to  be  an  inch  apart ;  and  that  of  the  threads  of 
the  other  only  one-third  of  an  inch  ;  what  will  be 
the  difference  of  the  advantage  gained  by  one  of 
the  screws  over  the  other  ? 

C.  The  one  whose  threads  are  three  times  nearer 
than  those  of  the  other,  must,  I  should  think,  give 
three  times  the  most  advantage. 

F .  Give  me  the  reason  for  what  you  assert. 
C.  Because,  from  the  principle  of  the  inclined 
plane,  I  learnt  that  if  the  height  of  two  planes  were 
the  same,  but  the  length  of  one  twice,  thrice,  or  four 
tmies  greater  than  that  of  the  other,  the  mechanical 
advantage  gained  by  the  longer  plane  would  be  two, 
three,  or  four  times  more  than  that  gained  by  the' 
shorter.  Now  in  the  present  case,  the  height  gained 
m  both  screws  is  the  same,  one  inch,  but  °the  space 
passed  in  that,  three  of  whose  threads  go  to  an  inch, 


OP  THB  SCREW.  69 
must  be  three  times  as  great  as  the  space  passed 
in  the  other  ;  therefore,  as  space  is  passed,  or  time 
lost,  just  in  proportion  to  the  advantage  gained,  I 
infer  that  three  times  more  advantage  is  gained  by 
the  screw,  the  threads  of  which  are  one-third  of  an 
inch  apart,  than  by  that  whose  threads  are  an  inch 
apart. 

F,  Your  inference  is  just,  and  naturally  follows 
from  an  accurate  knowledge  of  the  principle  of  the 
inclined  plane.  But  we  have  said  nothing  about  the 
lever. 

C.  This  seemed  hardly  necessary,  it  bemg  so  obvious 
to  any  one,  who  will  think  a  moment,  that  power  is 
gained  by  that  as  in  levers  of  the  first  kind,  according 
to  the  length  h  from  the  nut. 

F,  Let  us  now  calculate  the  advantage  gamed  by 
a  screw,  the  threads  of  which  are  half  an  inch  dis- 
tance from  one  another,  and  the  lever  7  feet  long. 

C.  I  think  you  once  told  me,  that  if  the  radms  of  a 
circle  were  given,  in  order  to  find  the  circumference  I 
must  multiply  that  radius  by  6. 

F.  I  did  ;  for  though  that  is  not  quite  enough,  yet 
it  will  answer  all  common  purposes,  till  you  are  a 
little  more  expert  in  the  use  of  decimals. 

C.  Well,  then,  the  circumference  of  the  circle 
made  by  the  revolution  of  the  lever  will  be  7  feet, 
multiplied  by  6,  which  is  42  feet,  or  504  inches; 
but,  during  this  revolution,  the  screw  is  raised  only 
half  an  inch,  therefore  the  space  passed  by  the  moving 
power  will  be  1008  times  greater  than  that  gone 
through  by  the  weight,  consequently  the  advantage 
gained  is  1008,  or  one  pound  applied  to  the  lever 
will  balance  1008  pounds  acting  against  the  screw. 

F.  You  perceive  that  it  follows  as  a  corollary 
from  what  you  have  been  saying,  that  there  are  two 
methods  by  which  you  may  increase  the  mechanical 
advantage  of  the  screw. 

C.  I  do  ;  it  may  be  done  either  by  takmg  a  longer 
lever,  or  by  diminishing  the  distance  of  the  threads  of 
the  screw. 


70  MECHANICS. 

F.  Tell  me  the  result  then,  supposing  the  threads 
of  the  screw  so  fine  as  to  stand  at  the  distance  of  but 
one  quarter  of  an  inch  asunder ;  and  that  the  length 
of  the  lever  were  8  feet  instead  of  7. 

C.  The  circumference  of  the  circle  made  by  the 
lever  will  be  8  multiplied  by  6,  equal  to  48  feet  or 
576  inches,  or  2304  quarter  inches,  and  as  the  eleva- 
tion of  the  screw  is  but  one  quarter  of  an  inch,  the 
space  passed  by  the  power  will,  therefore,  be  2304 
times  greater  than  that  passed  by  the  weight,  which  is 
the  advantage  gained  in  this  instance. 

F.  A  child,  then,  capable  of  moving  the  lever  suf- 
ficiently to  overcome  the.  friction,  with  the  addition  of 
a  power  equal  to  one  pound,  will  be  able  to  raise 
2304  pounds,  or  something  more  than  20  hundred 
weight  and  a  half.  The  strength  of  a  powerful  man 
would  be  able  to  do  20  or  30  times  as  much  more. 

C.  But  I  have  seen  at  IMr.  VVilmot's  paper-mills, 
to  which  I  once  v/ent,  six  or  eight  men  use  all  their 
strength  in  turning  a  screw,  in  ( rder  to  press  out  the 
water  of  the  newly-made  paper.  The  power  applied 
in  that  case  must  have  been  very  great  indeed. 

F.  It  was  ;  but  I  dare  say  that  you  are  aware  that 
it  cannot  be  estimated  by  multiplying  the  power  of 
one  man  by  the  numxber  of  men  employed. 

C.  That  is,  because  the  men  standing  by  the  side  of 
o«e  another,  the  lever  is  shorter  to  every  man  the 
nearer  he  stands  to  the  screw,  consequently,  though 
he  may  exert  the  same  strength,  yet  it  is  not  so  effec- 
tual in  moving  the  machine,  as  the  exertion  of  him 
who  stands  nearer  to  the  extremity  of  the  lever. 

jp.  The  true  method  therefore  of  calculating  the 
power  of  this  machine,  aided  by  the  strength  of  these 
men,  would  be  to  estimate  accurately  the  power  of 
each  man  according  to  his  position,  and  then  to  add 
all  these  separate  advantages  together  for  the  total 
power  gained. 

E.  A  machine  of  this  kind  is,  I  believe,  used  by 
bookbinders,  to  press  the  leaves  of  the  books  together 
before  they  are  stitched  ? 


OF  THE  PENDULUM.  Tl 
F  Yes  it  is  found  in  every  bookbinder's  work- 
shop, and  is  particularly  useful  where  persons  are  , 
desirous  of  having  small  books  reduced  to  a  stil 
smaller  size  for  the  pocket.  It  is  also  he  prmcipal 
machine  used  for  coining  money  ;  for  takmg  off  cop- 
per^plate  prints;  and  for  printmg  m  general.  Mr. 
Boulton  invented  a  magnificent  apparatus  for  coinmg  : 
the  whole  machinery  is  worked  by  an  improved  steam- 
endne,  which  rolls  the  copper  for  half-i)ence  ;  works 
the  screw-presses  for  cutting  out  the  circular  pieces 
of  copper,  and  coins  both  the  faces  and  edges  of  the 
money  at  the  same  time  :  and  since  the  circulation  of 
the  new  half-pence,  we  are  all  acquainted  with  the 
superior  excellence  of  the  workmanship.  By  this 
loachinery  four  boys,  of  ten  or  twelve  years  old,  are 
capable  of  striking  30,000  guineas  m  an  hour  and 
the  machine  itself  keeps  an  unerrmg  account  ot  the 

^^^E!^And?have  seen  the  cyder-press  in  Kent,  which 
consists  of  the  same  kind  of  machine. 

F  It  would,  my  dear,  be  an  almost  endless  task, 
were*  we  to  attempt  to  enumerate  all  the  purposes  to 
which  the  screw  is  applied  in  the  mechanical  arts  o 
life  -  it  will,  perhaps,  be  sufficient  to  tell  you,  that 
wheVever  great  pressure  is  required,  there  the  power 
of  the  screw  is  uniformly  employed. 

CONVERSATION  XXII. 

ON  THE  PENDULUM, 

E.  T  have  been  so  delighted  with  the  Conversa- 
tions you  have  permitted  us  to  have  with  you  my 
dear  papa,  that  I  can  scarcely  thmk  of  any  thmg 
but  what  you  have  been  explaining  to  us  ;  and  now 
when  I  see  a  machine  of  any  kind,  I  begin  to  examme 
the  various  combinations  of  levers,  wheels,  and  pulleys. 

F  I  am  very  glad  to  find  that  my  explanations  ot 
the  mechanical  powers  have  excited  your  curiosity 
so  much  ;  and  1  shall  always  feel  a  pleasure  m  com- 
municating any  thing  I  know. 

E.  I  am  very  much  obliged  to  you,  papa.  When 


T2  MECHANICS. 
Charles  and  I  were  down  stairs,  we  were  examining 
the  kitchen  clock  ;  we  saw  wheels  and  axles,  levers, 
screws,  pulleys,  &c.  but  neither  of  us  knew  what  to 
call  the  pendulum.    Is  that  a  mechanical  power  ? 

It  is  not  called  a  mechanical  power,  because  it 
does  not  convey  any  mechanical  advantage,  but  the 
theory  of  the  pendulum  depends  on  that  of  the  in- 
clined plane. 

-E.  What  is  meant  by  the  term  pendulum  ? 
F.  The.  name  is  applied  to  any  body  so  suspended 
that  it  may  swing  freely  backwards  and  forwards,  of 
which  the  great  law  is,  that  its  oscillations  are  always 
performed  in  equal  times  ;  and  it  is  this  remarkable 
property  which  makes  it  a  time-keeper.    A  common 
pendulum  consists  of  a  ball, 
as  a,  suspended  by  a  rod 
from  a  fixed  point,  at  b,  and 
made  to  swing  backwards 
and   forwards    under  this 
point.      The    ball  being 
raised  to  c,  and  then  set  at 
liberty,    falls   back    to  o, 
with  an  accelerating  motion, 
like  a  ball  rolling  down  a 
slope  (or  inclined  plane) ;  and  when  arrived  there. 
It  has  just  acquired  force  enough  to  carry  it  to  d,  at 
an  equal  elevation  on  the  other  side;  from  this  it 
falls  back  again,  again  to  rise,  and  v»'ould  so  continue 
on  for  ever,  if  there  was  no  impediment  either  from 
the  air  or  friction. 

C.  Are  the  laws  which  regulate  these  movements 
so  simple  that  we  can  understand  them  ? 

F .  I  think  they  are  ;  the  most  important  of  them 
ore  these  : — 1.  If  a  pendulum  vibrates  in  very  small 
circles,  the  times  of  vibration  may  be  considered  as 
equal,  whatever  be  the  proportion  of  the  circles. — 
2.  Pendulums  which  are  of  the  same  length  vibrate 
in  the  same  manner,  whatever  be  the  proportion  of  the 
V  eight  of  the  bob.— 3.  The  velocity  of  the  bob,  or 
I  all,  in  the  lowest  point,  will  be  sr,  the  length  of  tiie 
chord  of  the  arch  which  it  describes  in  its  descent.— 


OF  THE  PENDULUM. 


4.  The  times  of  vibration  of  different  pendulums  in 
similar  arches  are  proportional  to  the  isquare  roots  of 
their  different  lengths. — 5.  Hence  the  lengths  of  pen- 
dulums are  as  the  squares  of»the  times  of  vibrations, 
— 6.  In  the  latitude  of  London,  a  simple  pendulum 
vv^ill  depart  07ice  in  a  second  in  a  small  arch,  if  its 
length  be  39^  inches. 

C.  Does  the  length  of  a  pendulum  influence  the 
time  of  its  vibrations  ? 

F.  Yes  ;  long  pendulums  depart  more  slowly  than 
short  ones ;  because, in  corresponding  arcs,  or  paths,  the 
bob  or  ball  of  the  large  pendulum  has  a  greater  jour- 
ney to  perform,  without  having  a  steeper  line  of  descent. 

C.  If  I  understand  you  rightly,  a  pendulum  which 
describes  seconds  is  39i  inches  in  length,  and  the 
leftgths  are  as  the  square  roots  of  the  times  ;  therefore, 
the  length  of  a  half-second  pendulum  is  9|  inches, 
and  the  length  of  a  quarter-second  pendulum  will  be 
2|g  inches. 

F.  You  are  quite  correct. 

E.  But,  if  you  wished  the  pendulum  to  beat  longer 
than  one  second,  how  could  that  be  done'? 

F.  As  a  body  falls  four  times  as  fast  in  two  seconds 
as  m  one,  a  pendulum  must  be  four  times  as  long  to 
beat  once  in  two  seconds  as  to  beat  every  second. 

E.  How  is  it  made  to  denote  the  time  on  the  clocks  1 

F,  A  com.mon  clock  is  merely  a  pendulum  with 
wheel-work  attached  to  it  to  record  the  number  of 
vibrations.  The  wheels  shew  how  many  swings  of 
the  pendulum  have  taken  place,  because  at  every 
beat  a  tooth  of  the  last  wheel  is  allowed  to  pass ; 
now,  if  this  wheel  have  60  teeth,  it  will  just  turn 
round  once  for  60  beats  of  the  pendulum,  or  seconds, 
and  a  hand  fixed  on  the  axis  projecting  through  the 
dial-plate  will  be  the  second-hand  of  the  clock.  The 
other  wheels  are  so  arranged,  and  their  teeth  so  pro- 
portioned, that  one  turns  60  times  slower  than  the 
firsj,  to  fit  its  axis  to  carry  a  minute-hand,  and 
another,  by  moving  twelve  times  slower  still,  is  fitted 
to  carry  an  hour-hand. 


ASTRONOMY. 


CONVERSATION  I. 
OP  THE   FIXED  STARS. 

TUTOR  CHARLES  JAMES. 

Charles.  The  delay  occasioned  by  our  unusually 
long  walk  has  afforded  us  one  of  the  most  brilliant 
views  of  the  heavens  that  I  ever  saw. 

James.  It  is  uncommonly  clear,  and  the  longer  I 
keep  my  eyes  fixed  upwards  the  more  stars  seem  tc 
appear  :  how  is  it  possible  to  number  these  stars  1  and 
yet  I  have  heard  that  they  are  numbered,  and  ev%n 
arranged  in  catalogues  according  to  their  apparent 
magnitudes.  Pray,  sir,  explain  to  us  how  this  busi- 
ness was  performed. 

Tutor.  This  I  will  do,  with  great  pleasure,  some 
time  hence ;  but  at  present  I  must  tell  you,  that  in 
viewing  the  heavens  with  the  naked  eye,  we  are  very 
much  deceived  as  to  the  supposed  number  of  stars 
that  are  at  any  time  visible.  It  is  generally  admitted, 
and  on  good  authority  too,  that  there  are  never  more 
than  one  thousand  stars  visible  to  the  sight,  unassisted 
by  glasses,  at  any  one  time,  and  in  one  place. 

J.  What!  can  I  see  no  more  than  a  thousand 
stars  if  I  look  all  around  the  heavens  ?  I  should  sup- 
pose there  were  millions. 

T,  This  number  is  certainly  the  limit  of  what  you 
can  at  present  behold  ;  and  that  which  leads  you, 
and  persons  in  general,  to  conjecture  that  the  number 
is  so  much  laiger,  is  owing  to  an  optical  deception. 

J.  Are  we  frequently  liable  to  be  deceived  by  our 
senses  ? 

T.  We  are,  if  we  depend  on  them  singly:  but 
where  we  have  an  opportunity  of  calling  in  the  assist- 
ance of  one  sense  to  the  aid  of  another,  we  are  seldom 
subject  to  this  inconvenience. 


OF  THE  FIXED  STARS. 


75 


C.  Do  you  not  know,  that  if  you  place  a  small 
marble  in  the  palm  of  the  left  hand,  and  then  cross 
the  second  finger  of  the  right  hand  over  the  first,  and 
in  that  position,  with  your  eyes  shut,  move  the  marble 
with  those  parts  of  the  two  fingers  at  once  which  are 
not  accustomed  to  come  into  contact  with  any  object 
at  the  same  time,  that  the  one  marble  will  appear  to 
the  touch  as  two  1  In  this  instance,  without  the  as- 
sistance of  our  eyes,  we  should  be  deceived  by  the 
sense  of  feeling. 

T.  This  is  to  the  point,  and  shews  that  the  judg- 
ment formed  by  means  of  a  single  sense  is  not  always 
to  be  depended  upon. 

J,  I  recollect  the  experiment  very  well ;  we  had 
it  from  papa,  a  great  while  ago.  But  that  has  no- 
thing to  do  with  the  false  judgment  which  we  are  said 
to  form  about  the  number  of  stars. 

T.  You  are  right ;  it  does  not  immediately  concern 
the  subject  before  us,  but  it  may  be  useful  as  af- 
fording a  lesson  of  modesty,  by  instructing  us  that 
we  ought  not  to  close  our  minds  against  new  evi- 
dence that  may  be  offered  upon  any  topic,  notwith- 
standing the  opinions  we  may  have  already  formed. 
You  say  that  you  see  millions  of  stars,  whereas  the 
ablest  astronomers  assert,  that  with  the  naked  eye  you 
cannot  at  one  time  see  so  many  as  a  thousand. 

C.  I  should  indeed  have  thought  with  my  brother, 
had  you  not  asserted  the  contrary  :  and  I  am  anxious 
to  know  how  the  deception  happens,  for  I  am  sure 
there  must  be  a  great  deception  somewhere,  if  I  do 
not  at  this  time  behold  very  many  thousands  of  stars 
in  the  heavens. 

T.  You  know  that  we  see  objects  only  by  means 
of  the  rays  of  light  which  proceed  from  them  in  every 
direction.  And  you  must,  for  the  present,  give  me 
credit,  when  I  tell  you  that  the  distance  of  the  fixed 
stars  from  us  is  immensely  great ;  consequently  the 
rays  of  light  have  to  tiavel  this  distance,  in  the  course 
of  which,  especially  in  their  p?.ssage  through  our  at- 
mosphere, they  are  subject  to  numberless  rejiections 


7G  ASTRONOMY, 
and  refractions.  By  means  of  these,  other  rays  of 
light  come  to  the  eye,  every  one  of  which,  perhaps, 
impresses  upon  the  mind  the  idea  of  so  many  sepa- 
rate stars.  Hence  arises  that  optical  fallacy  by  which 
we  are  led  to  believe  the  stars  which  we  behold  are 
innumerable. 

J.  1  should  like  to  see  an  experiment  to  confirm 
this. 

r.  I  have  no  objection  : — in  every  case  you  ought 
to  require  the  best  evidence  that  the  subject  will  ad- 
mit of.  I  will  shew  you  two  experiments,  which  will 
go  a  good  way  to  remove  the  difficulty.  But,  for  this 
purpose,  we  must  step  into  the  house. 

Here  are  two  common  looking-glasses,  which,  phi- 
losophically speaking,  are  plane  mirrors.  I  place 
them  in  such  a  manner  on  the  table  that  they  sup- 
port one  another  from  falling  by  meeting  at  the 
tops.  I  now  place  this  half-crown  between  them,  on 
a  book,  to  raise  it  a  little  above  the  table.  'J'ell  me 
how  many  pieces  of  money  you  would  suppose 
there  were,  if  you  did  not  know  that  I  had  used  but 
one. 

/.  There  are  several  in  the  glasses. 

T.  I  will  alter  the  position  of  the  glasses  a  Ihtle, 
by  making  them  almost  parallel  to  one  another  ;  now 
look  into  them,  and  say  what  you  see. 

J.  There  are  more  half-crowns  now  than  there 
were  before. 

T.  It  is  evident,  then,  that  by  reflection  only,  a 
single  object,  for  I  have  made  use  of  but  one  half- 
crown,  will  give  you  the  idea  of  a  vast  number. 

C.  If  a  little  contrivance  had  been  used  to  conceal 
the  method  of  making  the  experiment,  I  sliould  not 
have  believed  but  that  there  had  been  several  half- 
crowns  instead  of  one. 

T.  Bring  me  your  multiplying  glass  ;  look  through 
it  at  the  candle  :  how  many  do  you  see  1  or  rather 
how  many  candles  should  you  suppose  there  were, 
did  you  not  know  that  there  was  but  one  on  the 
table  ? 


OF  THE  F1XET>  STARS.  77 
J.  A  great  many  \  and  a  pretty  sight  it  is. 
C.  Let  me  see ; — yes,  there  are  :  but  1  can  easily 
count  them  ;  there  are  sixteen. 

T.  There  will  be  just  as  many  images  of  the 
candle,  or  any  other  object  at  which  you  look,  as 
there  are  different  surfaces  on  your  glass.  For  by 
the  principle  of  refraction,  the  image  of  the  candle  is 
seen  in  as  many  different  places  as  the  glass  has  sur- 
faces ;  consequently,  if  instead  of  16  there  had  been 
60,  or,  if  they  could  have  been  cut  and  polished  so 
small,  600,  then  the  single  candle  would  have  given 
you  the  idea  of  60,  or  600.  What  think  you  now  about 
the  stars  ? 

J.  Since  I  have  seen  that  reflection  and  refraction 
will  each,  singly,  afford  such  optical  deceptions,  I 
can  no  longer  doubt  but  that,  if  both  these  causes 
are  combined,  as  you  say  they  are,  with  respect  to 
the  rays  of  light  coming  from  the  fixed  stars,  a  thou- 
sand real  luminaries  may  have  the  power  of  exciting 
in  my  mind  the  idea  of  millions. 

T,  I  will  mention  another  experiment,  for  which  you 
may  be  prepared  against  the  next  clear  starliglit  night. 
Get  a  long  narrow  tube,  the  longer  and  narrower  the 
better,  provided  its  weight  does  not  render  it  unma- 
nageable :  examine  through  it  any  one  of  the  largest 
fixed  stars,  which  are  called  stars  of  the  first  magni- 
tude, and  you  will  find  that,  though  the  tube  takes  in 
as  much  sky  as  would  contain  many  such  stars,  yet 
that  the  single  one  at  which  you  are  looking  is 
scarcely  visible,  by  the  few  rays  which  come  directly 
from  it :  this  is  another  proof  that  the  brilliancy  of 
the  heavens  is  much  more  owing  to  reflected  and  re- 
fracted light,  than  to  the  direct  rays  flowing  from  the 
stars. 


CONVERSATION  IL 

OF  THE   FIXED  STARS. 

C.  Another  beautiful  evening  presents  itself  j  shall 


78 


ASTRONOMY. 


we  take  the  advantage  which  it  offers  of  going  on 
with  our  astronomical  lectures  ? 

T.  I  have  no  objection,  for  we  do  not  always  enjoy 
such  opportunities  as  the  brightness  of  the  present 
evening  affords. 

J.  1  wish  very  much  to  know  how  to  distinguish 
the  stars,  and  to  be  able  to  call  them  by  their  proper 
names. 

T,  This  you  may  very  soon  learn  ;  a  few  evenmgs, 
well  improved,  will  enable  you  to  distinguish  all  the 
stars  of  the  first  magnitude  v/hich  are  visible,  and  all 
the  relative  positions  of  the  different  constellations. 

J.  What  are  constellations,  sir  ? 

r.  The  ancients,  that  they  might  the  better  distin- 
guish and  describe  the  stars,  with  regard  to  their  situ- 
ation in  the  heavens,  divided  them  into  constellations, 
that  is,  systems  of  stars,  each  system  consisting  of 
such  stars  as  were  near  to  each  other,  giving  them 
the  names  of  such  men  or  things  as  they  fancied  the 
space  which  they  occupied  in  the  heavens  represented. 

C.  Is  it  then  perfectly  arbitrary,  that  one  collection 
is  called  the  great  hear,  another  the  dragon,  a  third 
Hercules,  and  so  on? 

T.  It  is  ;  and  though  there  have  been  additions  to 
the  number  of  stars  in  each  constellation,  and  various 
new  constellations  invented  by  modern  astronomers, 
yet  the  original  division  of  the  stars  into  these  collec- 
tions was  one  of  those  few  arbitrary  inventions  which 
have  descended  without  alteration,  otherwise  than  by 
addition,  from  the  days  of  Ptolemy  down  to  the  pre- 
sent time.  Do  you  know  how  to  find  the  four  car- 
dinal points,  as  they  are  usually  called,  the  Aorth, 
South,  West,  and  East  ? 

J.  O  yes,  1  know  that  if  I  look  at  the  sun  at 
twelve  o'clock  at  noon,  I  am  also  looking  to  the 
South,  where  he  then  is ;  my  back  is  towards  the 
North  ;  the  West  is  on  my  right  hand,  and  the  East 
on  my  left. 

T,  But  you  must  learn  to  find  these  points  without 


OF  THE  FIXED  STARS.  T9 
the  assistance  of  the  sun,  if  you  wish  to  be  a  young 
astronomer.  t      /  * 

C  I  have  often  heard  of  the  north  pole  star;  that 
will  perhaps  answer  the  purpose  of  the  sun,  when  he 

^^T.'' You  are  right;  do  you  see  those  seven  stars 
which  are  in  the  constellation  of  the  Great,  Bear  ? 
some  people  have  supposed  their  position  will  aptly 
represent  a  plough;  others  say  that  they  are  more 
like  a  waggon  and  horses  ; — the  four 
stars  representing  the  body  of  a  wag- 
gon, and  the  other  three  the  hoises, 
and  hence  they  are  called  by  some 
the  plough,  and  by  others  they  are  ^ 
called  Charles's  wain  or  waggon.      >    '  u  ^  . 
Here  is  a  drawing  of  it ;  abdg  re-  ^  ^ 
present  the  four  stars,  and  e  2  b  the  " 
ether  three.  ' 
C.  What  is  the  star  p  ? 

T  That  represents  the  polar  star  to  which  you  just 
now  alluded;  and  you  observe,  that  if  a  line  were 
drawn  through  the  stars  h  and  a,  and  produced  tar 
enough,  it  would  nearly  touch  it. 

J.  Let  me  look  at  the  heavens  for  it  by  this  guide. 
There  it  is,  I  suppose;  it  shines  with  a  steady  and 
rather  dead  kind  of  light,  and  it  appears  to  me  that  it 
would  be  a  little  to  the  right  of  the  Ime  passmg 
throueh  the  stars  b  and  a. 

T.  It  would,  and  these  stars  are  generally  known 
by  the  name  of  the  pointers,  because  they  point  to  p, 
the  north  pole,  which  is  situated  a  little  more  than 
two  degrees  from  the  star  p. 

C.  Is  that  star  always  in  the  same  part  ot  the 
heavens'?  .        •  ^  •  • 

T  It  may  be  considered  as  uniformly  maintaining 
its  position,  while  the  other  stars  seem  to  move  round 
it  as  a  centre.  We  shall  have  occasion  to  refer  to 
this  star  again;  at  present,  I  have  directed  your  atten- 
tion to  it,  as  a  proper  method  of  finding  the  cardinal 
points  by  starlight. 


m  ASTRONOMY. 

J.  Yes,  I  understand  now,  that  if  I  look  to  the 
north,  by  standing  with  my  face  to  that  star,  the  south 
is  at  my  back,  on  my  right  hand  is  the  east,  and  the 
west  on  my  left. 

T.  This  is  one  important  step  in  our  astronomical 
studies  ;  and  we  can  make  use  of  these  stars  as  a  kind 
of  standard,  in  order  to  discover  the  names  and 
positions  of  others  in  the  heavens. 

C.  In  what  way  must  we  proceed  in  this  business  ? 

T.  I  will  give  you  an  example  or  two :  conceive  a 
line  drawn  from  the  star  z,  (Fig.  1,)  leaving  b  a  little 
to  the  left,  and  it  will  pass  through  that  very  brilliant 
star  near  the  horizon  towards  the  west. 

J.  I  see  the  star,  but  how  am  I  to  know  its  name  ? 

T.  Look  on  the  celestial  globe  for  the  star  z,  and 
suppose  the  line  drawn  on  the  globe,  as  we  conceived 
it  done  in  the  heavens,  and  you  will  find  the  star,  and 
its  name. 

C.  Here  it  is  ; — its  name  is  A  returns. 

r.  Take  the  figure,  (Fig.  1.)  and  place  Arcturus 
at  A,  which  is  its  relative  position,  in  respect  to  the 
constellation  of  the  Great  }3ear.  Now,  if  you  con- 
ceive a  line  drawn  through  the  stars  g  and  b,  and  ex- 
tended a  good  way  to  the  right,  it  will  pass  just  above 
another  very  brilliant  star.  Examine  the  globe  as 
before,  and  find  its  name. 

C.  It  is  Cape  11/ 1,  the  gont. 

T,  Now,  vvhtiiiever  you  see  any  of  these  stars,  you 
will  know  where  to  look  for  the  others  without  hesitation. 

J.  But  do  they  never  move  from  their  places  ? 

T.  With  respect  to  us,  they  seem  to  move  together 
with  the  whole  heavens.  But  they  always  remain  in 
the  same  relative  position,  with  respect  to  each  other. 
Hence  they  are  called /z'xeJ  stars,  in  opposition  to  the 
planets,  which,  like  our  earth,  are  continually  chano-- 
ing  their  places,  both  with  regard  to  the  fixed  stars, 
and  to  tiiemselves  also. 

C.  I  now  understand  pretty  well  the  method  of 
acquiring  a  knowledge  of  the  names  and  places  of  the 
stars. 


OF  THE  ECLIPTIC.  81 

T.  And  with  this,  we  will  put  an  end  to  our  present 
conversation. 

CONVERSATION  HI. 

OF  THE   FIXED   STARS,   AND  ECLIPTIC. 

T.  I  dare  say  that  you  will  have  no  difficulty  in 
finding  the  north  polar  star  as  soon  as  we  go  into  the 
open  air. 

J.  I  shall  at  once  know  where  to  look  for  that  and 
the  other  stars  which  you  pointed  out  last  night,  if 
they  have  not  changed  their  places. 

T.  They  always  keep  the  same  position,  with  respect 
to  each  other,  though  their  situation,  with  regard  to 
the  heavens,  will  be  different  at  different  seasons  of 
the  year,  and  in  different  hours  of  the  night.  Let  us 
go  into  the  garden. 

C.  The  stars  are  all  in  the  same  place  as  we  left 
them  last  evening.  Now,  Sir,  if  we  conceive  a 
straight  line  drawn  through  the  two  stars  in  the 
plough,  which,  in  your  figure,  (Fig.  1.)  are  marked 
d  and  g,  and  to  extend  a  good  way  down,  it  will  pass 
or  nearly  pass  through  a  very  bright  star,  though 
not  so  bright  as  Arcturus,  or  Capella;  what  is  that 
called  1 

T.  It  is  a  star  of  the  second  magnitude,  and  if  you 
refer  to  the  celestial  globe,  in  the  same  way  as  you 
were  instructed  last  night,  you  will  find  it  is  called 
Regulus,  or  Cor  heonis,  the  Lion's  Heart.  By  this 
method  you  may  quickly  discover  the  names  of  all 
the  principal  stars,  and  afterwards,  with  a  little 
patience,  you  will  easily  distinguish  the  others  which 
are  less  conspicuous. 

C.  But  they  have  not  all  names ;  how  are  they 
specified  ? 

T.  If  you  look  on  the  globe,  you  will  observe,  that 
tliey  are  distinguished  by  the  different  letters  of  the 
Greek  alphabet ;   and  in   those   constellations,  in 
which  there  are  stars  of  different  apparent  magnitudes^ 
E  2 


^2  ASTRONOMY, 
the  largest  is  «  alpha,  the  next  in  size  /3  beta,  the 
third  y  gamma,  the  fourth  d  delta,  and  so  on. 
J.  Is  there  any  particular  reason  for  this? 
T.  The  adoption  of  the  characters  of  the  Greek 
alphabet,  rather  than  any  other,  was  perfectly  arbi- 
trary; it  is,  however,  of  great  importance,  that  the 
same  characters  should  be  used  in  general  by  astro- 
nomers of  all  countries,  for  by  this  means  the  science 
is  in  possession  of  a  sort  of  universal  language. 
C.  Will  you  explain  how  this  is  ? 
T,  Suppose  an  astronomer  in  North  America,  Asia, 
or  any  other  part  of  the  earth,  observe  a  comet  in  that 
part  of  the  heavens  where  the  constellation  of  the 
Plough  h  situated,  and  he  wishes  to  describe  it  to  his 
friend  m  Great  Britain,  in  order  that  he  may  know, 
whether  it  was  seen  by  the  inhabitants  of  this  island.' 
For  this  purpose,  he  has  only  to  mention  the  time 
when  he  discovered  it ;  its  position,  as  nearest  to 
some  one  of  the  stars,  calling  it  by  the  Greek  letter  by 
which  it  is  designated  ;  and  the  course  which  it  took 
from  one  star  towards  another.    Thus  he  might  say, 
that  on  such  a  time  he  saw  a  comet  near  d  in  the 
Great  Bear,  and  that  its  course  was  directed  from  d 
to/3,  or  any  other  as  it  happens. 

C.  Then,  if  his  friend  here  had  seen  a  comet  at  the 
same  time,  he  would,  by  this  means,  know  whether  it 
w^as  the  same  or  a  different  comet  ? 

T,  Certainly;  and  hence  you  perceive  of  what  im- 
portance it  is  that  astronomers  in  different  countries 
should  agree  to  mark  the  same  stars,  and  systems  of 
stars,  by  the  same  characters.  But  to  return  to  that 
star,  to  which  you  just  called  my  attention,  the  Cor 
Lconis;  it  is  not  only  a  remai'kable  star,  but  its 
position  is  also  remarkable  :  it  is  situated  in  the 
Ecliptw. 

J.  What  is  that,  sir  ? 

T.  The  ecliptic  "is  an  imaginary'  circle  in  the 
heavens,  which  the  sun  appears  to  describe  in  the 
course  of  a  year.  If  you  look  on  the  celestial  alobe, 
you  will  see  it  marked  with  a  red  line,  perhaps  an 


OF  THE  ECLIPTIC.  83 

emblem  of  the  fierce  heat  communicated  to  us  by 
that  body. 

J.  But  the  sun  seems  to  have  a  circular  motion  m 
the  heavens  every  day  ? 

r.  It  does;  and  this  is  called  its  apparent  diurnal, 
or  daily  motion,  which  is  very  different  from  the  path 
it  appears  to  traverse  in  the  course  of  a  year.  The 
former  is  observed  by  the  most  inattentive  spectator, 
who  cannot  but  know  that  the  sun  is  seen  every 
morning  in  the  East,  at  noon  in  the  South,  and  in 
the  evening  in  the  West ;  but  the  knowledge  of  the 
latter  must  be  the  result  of  patient  observation.  ^ 

C.  And  what  is  the  ^reen  line  which  crosses  it  ? 

r.  It  is  called  the  Equator;  this  is  an  imaginary 
circle  belonging  to  the  earth,  which  you  must  take  for 
granted,  a  little  longer,  is  of  a  globular  form.  If  you 
can  conceive  the  plane  of  the  terrestrial  equator  to  be 
produced  to  the  sphere  of  the  fixed  stars,  it  would 
mark  out  a  circle  in  the  heavens,  called  the  celestial 
equator,  or  equinoctial,  which  would  cut  the  ecliptic  in 
two  parts ;  one  of  which  would  make  an  angle  with 
the  other  of  about  23|  degrees. 

J.  Can  we  trace  the  circle  of  the  ecliptic  m  the 
heavens  ? 

T.  It  may  be  done  with  tolerable  accuracy  by  two 
methods;  Jirst,  by  observing  several  remarkable 
fixed  starsj  to  which  the  moon  in  its  course  seems  to 
approach.  The  second  method  is  by  observing  the 
places  of  the  planets. 

C.  Is  the  moon  then  always  in  the  ecliptic  1 

T.  Not  exactly  so ;  but  it  is  always  either  in  the 
ecliptic,  or  within  five  degrees  and  a  third  of  it  on  one 
side  or  the  other.  The  planets  also,  by  which  I 
mean,  Mercury,  Venus,  Mars,  Jupiter,  Saturn,  and 
the  Herschel,  are  never  more  than  eight  decrees  dis- 
tant from  the  line  of  the  ecliptic. 

J.  How  can  we  trace  this  line,  by  help  of  the 
fixed  stars  ? 

T.  By  comparing  the  stars  in  the  heavens  with 
their  representatives  on  the  artificial  globe  a  practice 


84 


ASTRONOMY. 


which  may  be  readily  acquired,  as  you  have  seen.  I 
will  mention  to  you  the  names  of  those  stars,  and  you 
may  first  find  them  on  the  globe,  and  then  refer  to  as 
many  of  them  as  are  now  visible  in  the  heavens. 
The  first  is  in  the  Ram's  horn,  called  a  Arietis,  about 
ten  degrees  to  the  north  of  the  ecliptic  ;  the  second  is 
the  star  Aldeharan  in  the  BuWs  eye,  six  degrees  south 
of  the  ecliptic. 

C.  Then  if  at  any  time  I  see  these  two  stars,  I 
know  that  the  ecliptic  runs  between  them,  and  nearer 
to  Aldebaran,  than  to  that  in  the  Ram's  horn. 

T.  Yes  :  now  carry  your  eye  eastward  to  a  distance 
somewhat  greater  from  Aldebaran,  than  that  is  east  of 
a  Arietis,  and  you  will  perceive  two  bright  stars  at  a 
small  distance  from  one  another,  called  Castor  and 
Pollux  ;  the  lower  one,  and  that  which  is  least  bril- 
liant is  Pollux,  seven  degrees  on  the  north  side  of  the 
ecliptic.  Following  the  same  tract,  you  will  come  to 
Regains,  or  the  Cor  Leonis,  which  1  have  already  ob- 
served is  in  the  line  of  the  ecliptic.  Beyond  this,  and 
only  two  degrees  south  of  that  line,  you  will  find  the 
beautiful  star  in  the  virgin's  hand  called  Spica  Virginis, 
You  then  arrive  at  Antares,  or  the  Scorpion's  Heart, 
five  degrees  on  the  same  side  of  the  ecliptic.  After- 
wards you  will  find  cc  AqailcF.,  which  is  situated  nearly 
thirty  degrees  north  of  the  ecliptic  ;  and  farther  on  is 
the  star  Fomalhant  in  the  fish's  mouth,  about  as  many 
degrees  south  of  that  line.  The  ninth  and  last  of 
these  stars  is  Pegasus,  in  the  wing  of  the  flying- 
horse,  which  is  north  of  the  ecliptic  nearly  twenty 
degrees. 

/.  Upon  what  account  are  these  nine  stars  par- 
ticularly noticed  ? 

T.  They  are  selected  as  the  most  conspicuous  stars 
near  the  moon's  orbit,  and  are  considered  as  proper 
stations,  from  which  the  moon's  distance  is  calculated 
for  every  three  hours  of  time:  and  hence  are  con- 
structed those  tables  in  the  Nautical  Alwanac,  by 
means  of  which  navigators  in  tlieir  most  distant 
voyages  are  enabled  to  estimate,  on  the  trackless 


OF  THE  EPHEMERIS. 


85 


ocean,  the  particular  part  of  the  globe  on  which  they 
are. 

C.  What  do  you  mean  by  the  Nautical  Almanac  ? 

T.  It  is  a  kind  of  National  Almanac,  intended 
chiefly  for  the  use  of  persons  traversing  the  mighty 
ocean.  It  was  begun  in  the  year  1767,  by  Dr. 
Maskelyne,  the  Astronomer  Koyal  ;  and  is  published 
by  anticipation  for  several  years  beforehand,  for  the 
convenience  of  ships  going  out  upon  long  voyages. 
This  work  has  been  found  eminently  important  in  the 
course  of  the  late  voyages  round  the  world  for  making 
discoveries,  and  is  highly  useful  to  all  engaged  in 
navigation. 

CONVERSATION  IV. 

OF  THE  ErHEMEMS. 

C.  Your  second  method  of  tracing  the  ecliptic  was 
by  means  of  the  position  of  the  planets  :  will  you  ex- 
plain that  now  ? 

r.  1  will  ;  and  to  render  you  perfectly  qualified 
for  observing  the  stars,  I  will  devote  the  present  con- 
versation to  the  purpose  of  explaining  the  use  of 
White's  Ephemeris,  a  little  "book  which  is  published 
annually,  and  which  is  a  necessary  companion  to 
every  young  astronomer. 

J.  Must  we  understand  all  this  to  study  the  stars  1 

T,  You  must ;  or  some  other  book  of  the  same 
kind,  if  you  would  proceed  on  the  best  and  most 
rational  plan.  Besides,  when  you  know  the  use  of 
this  book,  which  you  will  completely  with  half  an 
hour's  attention,  you  have  nothing  more  to  do  in  order 
to  find  the  position  of  the  planets  at  any  day  of  the 
year,  than  to  turn  to  that  day  in  the  Ephemeris,  and 
you  will  instantly  be  directed  to  those  parts  of  the 
heavens,  in  which  the  different  planets  are  situated. 
Turn  to  the  second  page. 

C.  Here  the  astronomical  characters  are  explained. 

T,  The  first  twelve  are  the  representatives  of  the 


85  ASTRONOMY. 

signs  into  which  the  circle  of  the  ecliptic  is  divided, 

called  also  the  tw^elve  signs  of  the  Zodiac. 

<Y>  Aries.  ^  Leo.  f  Sagittarius. 

^  Taurus.  TT^  Virgo.  Capricorn, 

n  Gemini.  =5=  Libra.  CCC  Aquarius. 

25  Cancer.  ^1  Scorpio.  X  Pisces. 

In  astronomical  inquiries  every  circle  is  supposed 
to  be  divided  into  360  parts,  called  degrees,  and  since 
that  of  the  ecliptic  is  also  divided  into  12  signs,  each 
sign  must  contain  30  degrees.  Astronomers  subdi- 
vide each  degree  into  minutes  and  seconds  ;  thus, 
if  I  would  express  an  angle  of  25  degrees,  1 1  mi- 
nutes, and  45  seconds,  I  should  write  25^.  .1]/.  .45". 
Or,  if  I  would  express  the  situation  of  the  sun  for  the 
first  of  January,  1822,  I  look  into  the  Ephemeris  and 
find  it  in  Capricorn,  or  1^  10^.  .35'.  .48". 

J.  What  do  you  mean  by  the  Zodiac  1 

T.  It  is  a  broad  circle  or  belt  surrounding  the 
heavens,  about  sixteen  degrees  wide,  along  the  mid- 
dle of  which  runs  the  ecliptic.  The  term  Zodiac  is 
derived  from  a  Greek  word  signifying  an  animal,  be- 
cause each  of  the  twelve  signs  formerly  represented 
some  animal ;  that  which  we  now  call  Libra,  beino- 
by  the  ancients  reckoned  a  part  of  Scorpio. 

J.  Why  are  the  signs  of  the  Zodiac  called  by  the 
several  names  of  Aries,  Taurus,  Leo,  &c.?  I  see  no 
likeness  in  the  heavens  to  Rams,  or  Bulls,  or  Lions, 
which  are  the  English  words  for  those  Latin  ones. 

T.  jVor  do  I ;  nevertheless,  the  ancients  saw,  by 
the  help  of  a  strong  imagination,  a  similarity  between 
those  animals,  and  the  places  which  certain  systems  or 
stars  took  up  in  the  heavens,  and  gave  them  the  names 
which  have  continued  to  this  day^ 

C.  Perhaps  these  were  originally  invented  in  the 
same  way  as  we  sometimes  figure  to  our  imagination 
the  appearances  of  men,  beasts,  ships,  trees,  &c.  in 
the  flying  clouds  or  in  the  fire. 

T.  They  might  possibly  have  no  better  authority 


OF  THE  EPHEMERIS.  87 
for  their  origin.  At  any  rate  it  will  be  useful  for  you 
to  have  the  names  of  the  twelve  signs  in  your  memory, 
as  well  as  the  order  in  which  they  stand  :  I  will  there- 
fore repeat  some  lines  written  by  Dr.  Watts,  in  which 
they  are  expressed  in  English,  and  will  be  easily  re- 
membered : 

The  Ram,  the  Bull,  the  heavenly  Ttvins, 
And  next  the  Crab  the  Lio7i  shines, 

The  Virgi7i  and  the  Scales  ; 
The  Scorpion,  Archer,  and  Sea-Go«if, 
The  Man  that  holds  tlie  watering-pot. 

And  Fish  with  glittering  tails. 

C.  We  come  now  to  the  characters  placed  before 
the  planets. 

T,  These,  like  the  former,  are  but  a  kind  of  short- 
hand characters,  which  it  is  esteemed  easier  to  write 
than  the  names  of  the  planets  at  length.  They  are 
as  follow  : — 

Venus. 
Mercury. 
The  Moon. 
Ceres.  ~\ 

Pallas,  f  new  planets, 
Juno.  Cot  asteroids. 
Vesta.  3 

With  the  other  characters  you  have  no  need  to  trou- 
ble yourselves,  till  you  come  to  calculate  eclipses,  and 
construct  astronomical  tables,  a  labour  which  may  be 
deferred  for  some  years  to  come.  Turn  to  the  eighth 
page  of  the  Ephemeris. 

/.  Have  we  no  concern  with  the  intermediate  pages 
between  the  second  and  eighth  1 

T,  They  do  not  contain  any  thing  that  requires  ex- 
planation. In  the  eighth  page  after  the  common 
almanac  for  January,  the  two  first  columns  point 
out  the  exact  time  of  the  sun's  rising  and  setting  at 
London  :  thus  on  the  10th  day  of  January  he  rises  at 
58  minutes  after  7  in  the  morning,  and  sets  at  2 


The  Herschel,  or 

? 

Uranus, 

^ 

b 

Saturn. 

3) 

U 

Jupiter. 

$ 

Mars. 

$ 

e 

The  Earth. 

0 

The  Sua, 

s 

88 


ASTRONOMY. 


minutes  past  4  in  the  afternoon.    The  third  column 
gives  the  declination  of  the  sun. 
J.  What  is  that,  sir  ? 

T.  The  declination  of  the  sun,  or  of  any  heavenly 
body,  is  its  distance  from  the  imaginary  circle  in  the 
heavens,  called  the  equinoctial.  Thus  you  observe 
that  the  sun's  declination  on  the  first  of  January  is 
23^.  .3'  south  ;  or,  it  is  so  many  degrees  south  of  the 
imaginary  equator.  Turn  to  March  1822,  and  you 
will  see  that  between  the  20th  and  21st  days  it  is  in 
the  equator,  for  at  12  o'clock  at  noon  on  the  20th  it 
is  only  16'  south,  and  at  the  same  hour  on  the  21st  it 
is  8'  north  of  that  line  ;  and  when  it  is  in  the  equator, 
then  it  has  no  declination. 

C.  Do  astronomers  always  reckon  from  12  o'clock 
at  noon  ? 

T,  They  do  ;  and  hence  the  astronomical  day  be-* 
gins  12  hours  later  than  the  day  according  to  common 
reckoning  ;  and  therefore  the  declination,  longitude, 
latitude,  &c.  of  the  sun,  moon,  and  planets,  are 
always  put  down  for  12  o'clock  at  noon  of  the  day  to 
which  they  are  opposite.  Thus  the  sun's  declination 
for  the  17th  of  January  at  12  o'clock  is  20«.  .48' 
south. 

C.  Is  that  because  it  is  the  commencement  of  the 
astronomical  day,  though  in  common  life  it  be  called 
12  o'clock? 

T.  It  is.  The  three  next  columns  contain  the 
moon's  declination,  the  time  of  her  rising  and  setting, 
and  the  time  of  her  southing,  or  when  she  comes  to 
the  meridian  or  south  part  of  the  heavens. 

C.  Does  she  not  come  to  the  south  at  noon  as  well 
as  the  sun  ] 

T.  No  ;  the  moon  never  comes  to  the  meridian  at 
the  same  time  as  the  sun,  but  at  the  time  of  new  moon. 
And  this  circumstance  takes  place  at  every  new 
moon,  as  you  may  see  by  casting  your  eye  down  the 
several  columns  in  the  Ephemeris  which  relate  to  the 
moon's  southing. 

J.  What  do  you  say  of  the  column  which  is  marked 


OF  THE  EPHEMERIS.  89 
sometimes  clock  before  the  sun,  at  others  clock  after 
the  sun  ?  ,      ,  r  j 

T.  A  full  explanation  of  that  must  be  deterred  till 
we  come  to  speak  of  the  equation  of  time  ;  at  present 
it  will  be  sufficient  for  you  to  know  that  if  you  are  m 
possession  of  a  very  accurate  and  weli-regulated 
clock,  and  also  of  an  excellent  sun-dial,  they  will  be 
together  only  four  days  in  a  year  ;  now  this  column  m 
the  Ephemeris  points  out  how  much  the  clock  is  be- 
fore the  sun,  or  the  sun  before  the  clock,  for  every 
day  in  the  year.  On  twelfth-day,  1822,  for  mstance 
the  clock  is  faster  than  the  sun  by  6  minutes  and  7 
seconds ;  but  if  you  turn  to  May-day,  you  will  find 
that  the  clock  is  3'  2"  slower  than  the  sun. 

/.  What  are  the  four  days  in  the  year  when  the 
clock  and  dial  are  together  7 

T.  About  the  15th  of  April,  the  15th  of  June,  the 
1st  of  September,  and  Christmas-day. 

C.  By  this  table  then  we  may  regulate  our  clocks 
and  watches. 

J.  In  what  manner  ?  ,11 
C.  Examine  on  any  particular  day  the  clock  or 
watch,  and  dial  at  the  same  time,  say  12  0  clock,  and 
ob-^erve  whether  the  difference  between  them  answer 
to  the  difference  set  down  in  the  table  opposite  to 
the  day  of  observation.  Thus  on  the  12th  ol  March, 
1822,  the  clock  did  not  shew  true  time  unless  it  was 
10'  3"  before  the  dial,  or  when  the  dial  is  li 
o'clock  it  must  be  10'  .  .  3"  past  12  by  the  clock  or 

watch.  ^  ,  ^  ry^T  ^ 

r  Well  let  us  proceed  to  the  next  page,  l  tie 
three  first  short  columns,  relating  only  to  the  duration 
of  daylight  and  twilight,  require  no  explanation  ;  he 
fourth  we  shall  pass  over  for  the  present  ;  and  the 
remaining  five  give  the  latitude  of  the  planets. 

J.  What  do  you  mean  by  the  latitude  sir  ?  ^ 

r.  The  latitude  of  any  heavenly  body  is  its  dis- 
tance from  the  ecliptic  north  or  south,  fhe  atitude 
of  Venus,  on  new-year's  day  1822,  was  P  . .  1  south. 

a  Then  the  latitude  of  heavenly  bodies  has  the 


90 


ASTRONOMY. 


same  reference  to  the  ecliptic  that  declination  has  to 
the  equator  ? 
T.  It  has. 

J.  But  I  do  not  see  any  table  of  the  sun's  latitude. 
T,  I  dare  say  your  brother  can  give  you  a  reason 
for  this. 

C.  Since  the  latitude  of  a  heavenly  body  is  its  dis- 
tance from  the  ecliptic,  and  since  the  sun  is  always 
in  the  ecliptic,  therefore  he  can  have  no  latitude. 

T,  The  longitude  of  the  sun  and  planets  is  the  only 
thing-  in  this  page  that  remains  to  be  explained.  The 
longitude  of  a  heavenly  body  is  its  distance  from  the 
first  point  of  the  sign  Aries,  and  it  is  measured  on  the 
ecliptic.  It  is  usual,  however,  as  you  observe  in  the 
Ephemeris,  to  express  the  longitude  of  a  heavenly 
body  by  the  degree  of  the  sign  in  which  it  is.  In  this 
way  the  sun's  longitude  on  the  first  of  January,  1822, 
was  m  Capricorn  10°  .  .  35' .  .  48" ;  that  of  the  moon 
in  Aries,  17<* .  .  44'. 

C.  There  are  som.e  short  columns  at  the  bottom  of 
the  former  page  that  you  have  omitted. 

r.  The  use  of  these  will  be  better  understood  when 
we  come  to  converse  respecting  the  planets.* 


CONVERSATION  V. 

OF  THE  SOLAR  SYSTEM. 

T.  We  wdll  now  proceed  to  the  description  of  the 
Solar  Sijstern. 

J.  Of  what  does  that  consist,  sir? 

T,  It  consists  of  the  sun  and  planets,  with  their 
satellites,  or  moons.  It  is  called  the  Solar  Sijstem 
from  Sol,  the  sun,  because  the  sun  is  supposed  to  be 
fixed  in  the  centre,  while  the  planets,  and  our  earth 
among  them,  revolve  round  him  at  different  distances. 

*  For  the  explanation  of  Heliocentric  Longitude,  see 
Conversation  XX. 


OF  THE  SOLAR  SYSTEM.  91 

C.  Bat  are  there  not  some  people  who  beUeve 
that  the  sun  goes  round  the  earth '] 

T.  Yes,  it  is  an  opinion  embraced  by  the  generahty 
of  persons,  not  accustomed  to  reason  on  these  subjects. 
It  was  adopted  by  Ptolemy,  who  supposed  the  earth 
perfectly  at  rest,  and  the  sun,  planets,  and  fixed 
stars,  to  revolve  about  it  every  twenty-four  hours. 

J.  And  is  not  that  the  most  natural  supposition  ? 

T.  If  the  sun  and  stars  were  small  bodies  in  com- 
parison of  the  earth,  and  were  situated  at  no  very 
great  distance  from  it,  then  the  system  maintained  by 
Ptolemy  and  his  followers  might  appear  the  most 
probable. 

/.  Are  the  sun  and  stars  very  large  bodies,  then  I 
T,  The  sun  is  more  than  a  miUion  of  times  larger 
than  the  earth  which  we  inhabit,  and  many  of  the 
fixed  stars  are  probably  much  larger  than  he  is. 

C.  What  is  the  reason,  then,  that  they  appear  so 
small  ? 

T.  This  appearance  is  caused  by  the  immense  dis- 
tance there  is  between  us  and  these  bodies.  It  is 
known  with  certainty  that  the  sun  is  more  than  95 
millions  of  miles  distant  from  the  earth,  and  the  near- 
est fixed  star  is  probably  more  than  two  hundred 
thousand  times  further  from  us  than  even  the  sun 
himself.* 

C.  But  we  can  form  no  conception  of  such  dis- 
tances. 

r.  We  talk  of  millions,  with  as  much  ease  as  of 
hundreds  or  tens,  but  it  is  not,  perhaps,  possible  for 
the  mind  to  form  any  adequate  conceptions  of  such 
high  numbers.  Several  methods  have  been  adopted 
to  assist  the  mind  in  comprehending  the  vastness  of 

*  The  youn^  reader  will,  when  he  is  able  to  manage 
the  subject,  see  this  clearly  demonstrated  by  a  series  of 
propositions  in  the  5th  book  of  Dr.  Enfield's  Institutes 
of  Natural  Philosophy.  Second  Edition.  See  p.  346,  to 
the  end  of  book  V. 


^2  ASTRONOMY, 
these  distances.    You  have  some  idea  of  the  swiftness 
with  which  a  cannon-ball  proceeds  from  the  mouth 
of  the  gun  ? 

1  have  heard  at  the  rate  of  eight  miles  in  a 
minute. 

T.  And  you  know  how  many  minutes  there  are  in 
a  year  ? 

/.  I  can  easily  find  that  out,  by  multiplying  365 
days  by  24  for  the  number  of  hours,  and  that  product 
by  60,  and  I  shall  have  the  number  of  minutes  in  a 
year,  which  number  is  525,600. 

r.  Now  if  you  divide  the  distance  of  the  sun  from 
the  earth  by  the  number  of  minutes  in  a  year,  multi- 
plied by  8,  because  the  cannon-bail  travels  at  the 
rate  of  8  miles  in  one  minute,  you  will  know  how 
long  any  body  issuing  from  the  sun,  with  the  velocity 
of  a  cannon-ball,  would  employ  in  reachiutr  the 
earth.  ^ 

C.  If  I  divide  95,000,000  by  525,600,  multiplied 
by  8,  or  4,204,800,  the  answer  will  be  more  than  22 
tlie^  number  of  years  taken  for  the  journey.  ' 

T.  Is  It  then  probable  that  bodies  so  large,  and  at 
such  distances  from  the  earth,  should  revolve  round 
it  every  day  ? 

C.  I  do  not  think  it  is.— Will  you,  sir,  go  on  with 
the  description  of  the  Solar  Sqsteiii  ? 

T,  According  to  this  system,  the  sun  is  in  the 
centre,  about  which  the  planets  revolve  from  wea  to 
east,  according  to  the  order  of  the  signs  in  the  ecliptic  • 
Uiat  IS,  if  a  planet  is  seen  in  Aries,  it  advances  to' 
1  aurus,  then  to  Gemini,  and  so  on. 

J.  Hov/  many  planets  are  there  belongino-  to  the 
sun  ?  °  ° 

T,  There  are  seven,  besides  some  smaller  bodies 
discovered  during  the  present  century.  C  is  the  sun' 
the  nearest  to  which  iUercurv  revolves  in  the  circle 
«  ;  next  to  him  is  the  beautiful  planet  Venus,  who 
pcrtorms  her  revolution  in  the  circle  h  ;  then  comes 
tiie  Earth  m  t;  next  to  which  b  Mars  in  e;  then 


OF  THE  SOLAR  SYSTEM.  9S 
Jupiter  in  the  circle  f;  afterwards  Saturn  in  g  ;  and 
far  beyond  him  the  Herschet  plar.et  performs  his  revo- 
hiiion  in  the  circle  h. 


J,  For  what  are  the  smaller  circles  which  are 
attached  to  several  of  the  larger  ones  intended  ? 

T,  They  are  intended  to  represent  the  orbits  of  the 
several  satellites  or  moons  belonging  to  some  of  the 


J.  What  do  you  mean  by  the  word  orbit  I 
T.  The  path  described  by  a  planet  in  its  course 
round  the  sun,  or  by  a  moon  round  its  primary  planet, 
is  called  its  orbit.  Look  to  the  orbit  of  the  earth  in  t, 
and  you  will  see  a  little  circle,  which  represents  the 
orbit  in  which  our  moon  performs  its  monthly  journey. 
»     C.  Has  neither  Mercury  nor  Venus  any  moonl 

T,  None  have  ever  been  discovered  belonging 
either  to  Mercury,  Venus,  or  Mars.  J upiter,  as  you 
observe  by  the  figure,  has  four  moons :  Saturn  has 
seven  :  and  the  Herschel  planet  (which  also  goes  by 
the  name  of  Uranus)  has  six,  which,  for  want  of  room, 
are  not  drawn  in  the  plate. 


94 


ASTRONOMY. 


C.  The  Solar  Si^slem,  then,  consists  of  the  sun  as  a 
centre,  round  which  revolve  seven  planets,  and  eighteen 
satellites  or  moons.  Are  there  no  other  bodies  be- 
longing to  it  ? 

T.  Yes,  four  other  planetary  bodies  have  been 
very  lately  discovered  as  belonging  to  the  solar  sys- 
tem. These  are  very  small,  and  called  the  Piazzi, 
Olbers,  &c.  from  the  gentlemen  v^ho  discovered  them. 
They  are  likewise  called  Ceres  Ferdincmdea,  Pallas, 
Juno,  and  Vesta.  There  are  comets  also  which  make 
their  appearance  occasionally  ;  and  it  would  be  wrong 
positively  to  affirm  that  there  can  be  no  other  planets 
belonging  to  the  Solar  System  ;  since,  besides  the 
four  bodies  just  mentioned,  it  is  only  within  these  few 
years  that  the  seventh,  or  the  Herschel,  has  been 
known  to  exist  as  a  planet  connected  vvith  this  system. 

C.  Who  first  adopted  the  system  of  the  world  which 
you  have  been  describing  ? 

T.  It  was  conceived  and  taught  by  Pythagoras  to  his 
disciples  600  years  before  the  time  of  Christ.  But  it 
seems  soon  to  have  been  disregarded,  or  perhaps 
totally  rejected  till  about  300  years  ago,  when  it  was 
revived  by  Copernicus,  and  is  at  length  generally 
adopted  by  men  of  science. 


CONVERSATION  VI. 

OF  THE  FIGURE  OF  THE  EARTH. 

T.  Having,  in  our  last  conversation,  given  you  a 
description  of  the  Solar  System  in  general,  we  will 
now  proceed  to  consider  each  of  its  parts  separately  : 
and  since  we  are  most  of  all  concerned  with  the 
earth,  we  will  begin  with  that  body. 

J .  You  promised  to  give  us  some  reasons  why  this 
earth  must  be  in  the  form  of  a  globe,  and  not  a  m.ere 
extended  plane,  as  it  appears  to  common  observation. 

T,  Suppose  you  were  standing  by  the  sea-shore, 
on  a  level  with  the  water,  and  at  a  very  considerable 
distance,  as  far  as  the  eye  can  reach,  you  observe  a 


OF  THE  FIGURE  OF  THE  EARTH.  95 
sbip  approaching,  what  ought  to  be  the  appearance, 
supposing  the  surface  of  the  sea  to  be  a  flat  plane  ? 

C.  We  should,  I  think,  see  the  whole  ship  at 
once,  that  is,  the  hull  would  be  visible  as  soon  as  the 
top-mast. 

T.  It  certainly  must,  or  indeed  rather  sooner,  be- 
cause the  body  of  the  vessel  being  so  much  larger  than 
a  slender  mast,  it  must  necessarily  be  visible  at  a 
greater  distance. 

J.  Yes,  I  can  see  the  steeple  of  a  church  at  a  much 
greater  distance  than  I  can  discern  the  iron  conductor 
which  is  upon  it,  and  that  I  can  perfectly  see  long 
before  the  little  piece  of  gold  wire,  which  is  fixed  at 
its  extremity,  is  visible. 

T.  Well,  but  the  top-mast  of  a  vessel  at  sea  is 
always  in  view  some  little  time  before  the  hull  of  the 
vessel  can  be  discerned.  Now,  if  the  surface  of  the 
sea  be  globular,  this  ought  to  be  the  appearance,  be- 
cause the  protuberance  or  swelling  of  the  water  be- 
tween the  vessel  and  the  eye  of  the  spectator  will 
hide  the  body  of  the  ship  some  time  after  the  pendant 
is  seen  above. 

C.  In  the  same  way  as  if  a  high  building,  a  church 
for  instance,  were  situated  on  one  side  of  a  hill,  and  I 
was  walking  up  on  the  opposite  side,  the  steeple  would 
come  first  in  sight,  and  as  I  advanced  towards  the 
summit,  the  other  parts  v/ould  come  successively  in 
view. 

T.  Your  illustration  is  quite  to  the  purpose  :  m  the 
same  way,  two  persons  walking  up  a  hill  on  the  oppo- 
site sides,  will  perceive  each  other's  heads  first ;  and 
as  they  advance  to  the  top,  the  other  parts  of  their 
bodies  will  become  visible.  With  respect  to  the  ship, 
the  following  figure  will  convey  the  idea  very  com- 
pletely. Suppose  cha  represent  a  small  part  of  the 
curved  surface  of  the  sea  ;  if  a  spectator  stand  at  a 
while  a  ship  is  at  c,  only  a  small  part  of  the  mast  is 
visible  to  him,  but,  as  it  advances,  more  of  the  ship  is 
seen,  till  it  arrive  at  e,  when  the  whole  will  be  in  sight. 


96 


ASTRONOMY. 


Fig.  3. 


C.  When  I  stood  by  the  sea  side  the  water  did  not 
appear  to  me  to  be  curved. 

i .  Perliaps  not ;  but  its  convexity  may  be  dis- 
covered upon  any  still  water,  as  upon  a  river,  which 
k  extended  a  mile  or  two  in  length ;  for  you  might  see  a 
very  small  boat  at  that  distance  while  standing  upright; 
if  then  you  stoop  down  so  as  to  bring  your  eye  near 
the  water,  you  will  find  the  surface  of  it  rising  in  such 
a  manner  as  to  cover  the  boat,  and  intercept  its  view 
completely.  Another  proof  of  the  globular  figure  of 
the  earth  is,  that  it  is  necessary  for  those  who  are  em- 
ployed in  cutting  canals,  to  make  a  certain  allowance 
for  the  convexity  ;  since  the  true  level  is  not  a  straight 
line,  but  a  curve  which  falls  below  it  eight  inches  in 
every  mile. 

C.  I  have  heard  of  people  sailing  round  the  world, 
which  is  another  proof,  I  imagine,  of  the  globular 
figure  of  the  earth. 

T,  It  is  a  well-known  fact  that  navigators  have  set 
out  from  a  particular  port,  and  by  steering  their 
course  continually  westward,  have  at  length  arrived 
at  the  same  place  from  whence  they  first  departed. 
Now  had  the  earth  been  an  extended  plane,  the 
longer  they  had  travelled  the  farther  must  they  have 
been  from  home. 

C.  How  is  it  known  that  they  continued  the  same 
course  1  might  they  not  have  been  driven  round  at 
open  sea? 

T.  By  means  of  the  mariner's  compass,  the  history, 
properties,  and  uses  of  which  1  will  explain  very  par- 
ticularly in  a  future  j)art  of  our  lectures,  the  method 


OF  THE  FIGUUE  OF  THE  EARTH»  97 
of  sailing  on  the  ocean  by  one  certain  tract,  is  as  sure 
as  travelling  on  the  high  London  road  from  the  me- 
tropolis to  York.  By  this  method,  Ferdinand  Magel- 
lan sailed,  in  the  year  1519,  from  the  western  coast  of 
Spain,  and  continued  his  voyage  in  avi^estward  course 
till  he  arrived  after  1124  days  in  the  same  port  from 
whence  he  set  out.  The  same,  with  respect  to  Great 
Britain,  was  done  by  our  own  countrymen  Sir  Francis 
Drake,  Lord  Anson,  Captain  Cook,  and  many  others, 

C.  Is  then  the  common  terrestrial  globe  a  just 
representation  of  the  earth  1 

T.  It  is,  with  this  small  difference,*  that  the  artifi- 
cial globe  is  a  perfect  sphere,  whereas  the  earth  is  a 
spheroid,  that  is,  in  the  shape  of  an  orange,  the  diame- 
ter from  pole  to  pole  being  about  37  miles  shorter  than 
that  at  the  Equator, 

J.  What  are  the  poles,  sir  1 

T.  In  the  artificial  globe 
there  is  an  axis  ns  about 
which  it  turns  ;  now  the 
two  extremities  or  ends  of 
this  axis  n  and  s  are  called 
the  poles. 

C.  Is  there  any  axis  be- 
longing to  the  earth  ? 

T.  No  ;  but,  as  we  shall 
to-morrow  shew,  the  earth 
turns  round  once  in  every 
24  hours  ;  so  astronomers 
imagine  an  axis  upon  which  it  revolves  as  upon  a 
centre,  the  extremities  of  which  imaginary  axis  are 
the  poles  of  the  earth  ;  of  these,  n,  the  north  pole, 
points  at  all  times  exactly  to  the  north  pole  of  the 


*  What  the  earth  loses  of  its  sphericity,  by  mountains 
and  valleys,  is  very  inconsiderable  ;  the  highest  moun- 
tain bearing  so  little  proportion  to  its  bulk,  as  scarcely 
to  be  equivalent  to  the  minutest  protuberance  on  the 
surface  of  an  orange. 


F 


98  ASTRONOMY. 

heavens,  whieh  we  have  already  described,  and  which 
is,  as  you  recollect,  within  two  degrees  of  the  polar 
star.   (See  Fig.  1.  p.  79.) 

/.  And  how  do  you  define  the  equator  ? 

T.  The  equator  ab  (Fig.  4.)  is  the  circumference 
of  an  imaginary  circle  passing  through  the  centre 
of  the  earth,  perpendicular  to  the  axis  ns,  and  at  equal 
distances  from  the  poles. 

C.  And  I  think  you  told  us,  that  if  we  conceived 
this  circle  extended  every  way  to  the  fixed  stars  it 
would  form  the  celestial  equator, 

T.  I  did  ;  it  is  also  called  the  equinoctial,  and  you. 
must  not  forget,  that  in  this  case  it  would  cut  the 
circle  of  the  ecliptic  cd  in  two  points. 

/.  Why  is  the  ecliptic  marked  on  the  terrestrial 
globe,  since  it  is  a  circle  peculiar  to  the  heavens  ? 

T.  Though  the  ecliptic  be  peculiar  to  the  heavens, 
and  the  equator  to  the  earth,  yet  they  are  both  drawn 
on  the  terrestrial  and  celestial  globes,  in  order, 
among  other  things,  to  shew  the  position  which  these 
imaginary  circles  have  to  one  another. 

I  shall  now  conclude  our  present  Conversation, 
with  observing,  that  besides  the  proofs  adduced  for  the 
globular  form  of  the  earth,  there  are  others  equally 
conclusive,  which  will  be  better  understood  a  few 
days  hence. 


CONVERSATION  YII. 

OF  THE  DIURNAL  MOTION  OF  THE  EARTH. 

T.  Well,  gentlemen,  are  you  satisfied  that  the  earth 
on  which  you  tread  is  a  globular  body,  and  not  a 
mere  extended  plane  ? 

C.  Admitting  the  facts  which  you  mentioned  yester- 
day, viz.  that  the  topmast  of  a  ship  at  sea  is  always 
visible  before  the  body  of  the  vessel  comes  into  sight ; 
that  navigators  have  repeatedly,  by  keeping  the  same 


DIURNAL  MOTION  OF  THE  EARTH.  99 
direction,  sailed  round  the  world ;  and  that  persons 
employed  in  digging  canals  can  only  execute  their 
work  with  effect  by  allowing  for  the  supposed  globu- 
lar shape  of  the  earth,  it  is  evident  the  earth  cannot  be 
a  mere  extended  plane. 

/.  But  all  these  facts  can  be  accounted  for  upon 
the  supposition  that  the  earth  is  a  globe,  and  there- 
fore you  conclude  it  is  a  globe  :  this  was,  1  believe, 
the  nature  of  the  proof? 

r.  It  was  :  let  us  now  advance  one  step  farther, 
and  shew  you  that  this  globe  turns  on  an  imaginary 
axis  every  twenty-four  hours;  and  thereby  causes  the 
succession  of  day  and  night. 

J.  I  shall  wonder  if  you  are  able  to  afford  such 
satisfactory  evidence  of  the  daily  motion  of  the  earth, 
as  of  its  globular  form. 

T.  I  trust,  nevertheless,  that  the  arguments  on  this 
subject  will  be  sufficiently  convincing,  and  that  be- 
fore we  part  you  will  admit,  that  the  apparent  mo- 
tion of  the  sun  and  stars  is  occasioned  by  the  diurnal 
motion  of  the  earth. 

C.  1  shall  be  glad  to  hear  how  this  can  be  proved  ; 
for  if,  in  the  morning,  I  look  at  the  sun  when  rising, 
it  appears  in  the  east,  at  noon  it  has  travelled  to  the 
south,  and  in  the  evening  I  see  it  set  in  the  western 
part  of  the  heavens. 

J.  Yes,  and  we  observed  the  same  last  night 
(March  the  1st)  with  respect  to  Arctnrus ;  for  about 
eight  o'clock  it  had  just  risen  in  the  north-east  part  of 
the  heavens,  and  when  we  went  to  bed,  two  hours 
after,  it  had  ascended  a  good  height  in  the  heavens, 
evidently  travelling  towards  the  west. 

T.  It  cannot  be  denied  that  the  heavenly  bodies 
appear  to  rise  in  the  east  and  set  in  the  west ;  but  the 
appearance  will  be  the  same  to  us,  whether  those 
bodies  revolve  about  the  earth  while  that  stands  still, 
or  they  stand  still  while  the  earth  turns  on  its  axis  the 
contrary  way. 

C.  Will  you  explain  this,  sir  1 


100 


ASTRONOMY. 


T.  Suppose  grch  to  represent  the 
earth,  t  the  centre  on  v/hich  it  turns 
from  west  to  east,  according  to  the 
order  of  the  letters  grcb.  If  a  spec- 
tator on  the  surface  of  the  earth  at  r, 
see  a  star  at  h,  it  v^ill  appear  to  him 
to  have  just  risen ;  if  now  the  earth  be 
supposed  to  turn  on  its  axis  a  fourth 
part  of  a  revolution,  the  spectator  will  be  carried  from 
r  to  c,  and  the  star  will  be  just  over  his  head ;  when 
another  fourth  part  of  the  revolution  is  completed  the 
spectator  will  be  at  b,  and  to  him  the  star  at  h  will 
be  setting,  and  will  not  be  visible  again  till  he  arrive, 
by  the  rotation  of  the  earth,  at  the  station  r. 

C.  To  the  spectator,  then,  at  r  the  appearance 
would  be  the  same  whether  he  turned  with  the  earth 
into  the  situation  h,  or  the  star  at  h  had  described,  in 
a  contrary  direction,  the  space  hzo  in  the  same  time. 

T.  It  certainly  would. 

J.  But  if  the  earth  really  turned  on  its  axis,  should 
we  not  perceive  the  motion  1 

T.  The  earth  in  its  diurnal  rotation  being  subject 
to  no  impediments  by  resisting  obstacles,  its  motion 
cannot  affect  the  senses.  In  the  same  way  ships  on 
a  smooth  sea  are  frequently  turned  entirely  round  by 
the  tide,  without  the  knowledge  of  those  persons  who 
happen  to  be  busy  in  the  cabin,  or  between  the 
decks. 

C.  That  is,  because  they  pay  no  attention  to  any 
other  object  but  the  vessel  in  which  they  are,  every 
part  of  which  moves  with  themselves. 

J.  But  if  while  the  ship  is  turning,  without  their 
knowledge,  they  happen  to  be  looking  at  fixed  distant 
objects,  what  will  be  the  appearance 

T.  To  them,  those  objects  which  are  at  rest  will 
appear  to  be  turning  round  the  contrary  way.  In 
tlie  same  manner  we  are  deceived  in  the  motion  of 
the  earth  round  its  axis,  for,  if  we  attend  to  nothing 
but  what  is  connected  with  the  earth,  we  cannot  per- 
ceive a  motion  of  which  we  partake  ourselves,  and  if 


DIURNAL  MOTION  OF  THE  EARTH.  101 
we  fix  our  eyes  on  the  heavenly  bodies,  the  motion  of 
the  earth  being  so  easy,  they  will  appear  to  be  turn- 
ing in  a  direction  contrary  to  the  real  motion  of  the 
earth. 

C.  I  have  sometimes  seen  a  skylark  hovering  and 
singing  over  a  particular  field  for  several  minutes  toge- 
ther ;  now,  if  the  eartii  is  continually  in  motion  while 
the  bird  remains  in  the  same  part  of  the  air,  why  do 
we  not  see  the  field,  over  which  he  first  ascended, 
pass  from  under  him  1 

r.  Because  the  atmosphere  in  which  the  lark  is 
suspended  is  connected  with  the  earth,  partakes  of 
its  motion,  and  carries  the  lark  along  with  it ;  and 
therefore,  independently  of  the  motion  given  to  the 
bird  by  the  exertion  of  its  wings,  it  has  another  in 
common  with  the  earth,  yourself,  and  all  things  on 
it,  and  being  common  to  us  all,  we  have  no  methods 
of  ascertaining  it  by  means  of  the  senses. 

J.  Though  the  motion  of  a  ship  cannot  be  observed 
vdthout  objects  at  rest  to  compare  with  it,  yet  I  can- 
not help  thinking  that  if  the  earth  moved  we  should 
be  able  to  discover  it  by  means  of  the  stars,  if  they 
are  fixed. 

r.  Do  you  not  remember  once  sailmg  very  swiftly 
on  the  river,  when  you  told  me  that  you  thought  all 
the  trees,  houses,  &c.  on  its  banks  were  in  motion  ? 

J.  I  now  recollect  it  well ;  and  I  had  some  diffi- 
culty in  persuading  myself  that  it  was  not  so. 

C.  This  brings  to  my  mind  a  still  stronger  decep- 
tion of  this  sort :  when  travelling  with  great  speed  in 
a  post-chaise,  I  suddenly  waked  from  a  sleep  in  a 
smooth  but  narrow  road,  and  I  could  scarcely  help 
thinking,  for  several  minutes,  but  th-at  the  trees  and 
hedges  were  running  away  from  us,  and  not  we  from 
them. 

r.  I  will  mention  another  curious  instance  of  this 
kind  :  if  you  ever  happen  to  travel  pretty  swiftly  in  a 
carriage,  by  the  side  of  a  field  ploughed  into  long  nar- 
row ridges,  and  perpendicular  to  the  road,  you  will  think 
that  all  the  ridges  are  turning  round  in  a  direction  con- 


10^  ASTRONOMY, 
trary  to  that  of  the  carriage.    These  facts  may  satisfy 
you  that  the  appearances  will  be  precisely  the  same 
to  us,  whether  the  earth  turn  on  its  axis  from -west  to 
easts  or  the  sun  and  stars  move  from  east  to  west. 

J.  They  will ;  but  which  is  the  more  natural  con- 
clusion ? 

T.  This  you  shall  determine  for  yourself.  If  the 
earth  (Fig,  4.)  turn  on  its  axis  in  24  hours,  at  what 
rate  will  any  part  of  the  equator  ab  move  I 

C.  To  determine  this  we  must  find  the  measure  of 
its  circumference,  and  then  dividing  this  by  24,  we 
shall  get  the  number  of  miles  passed  through  in  an 
hour. 

T,  Just  so  :  now  call  the  semi-diameter  of  the 
earth  4000  miles,  which  is  rather  more  than  the  true 
measure. 

/.  Multiplying  this  by  six  *  will  give  24,000  miles 
for  the  circumference  of  the  earth  at  the  equator,  and 
this  divided  by  24,  gives  1000  miles  for  the  space 
passed  through  in  an  hour,  by  an  inhabitant  of  the 
equator. 

r.  You  are  right.  The  sun,  I  have  already  told 
you,  is  95  millions  of  miles  distant  from  the  earth  ; 
tell  me,  therefore,  Charles,  at  what  rate  that  body 
must  travel  to  go  round  the  earth  in  24  hours  1 

C,  I  will :  95  millions  multiplied  by  six  will  give 
910  millions  of  miles  for  the  length  of  his  circuit ; 
this  divided  by  24  gives  nearly  24  millions  of  miles 
for  the  space  he  must  travel  in  an  hour,  to  go  round 
in  a  day. 

•  If  the  reader  would  be  accurate  in  his  calculations, 
he  must  take  the  mean  radius  of  the  earth  at  39G5  miles, 
and  this,  multiplied  hy  C2S,31S  will  give  24,912  miles  for 
the  circumference.  Through  the  remainder  of  this  work, 
the  decimals  in  multiplication  are  omitted,  in  order  that 
the  mind  may  not  be  burdened  with  odd  numbers.  It 
seemed  necessary,  however,  in  this  place  to  give  the  true 
serai-diameter  of  the  earth,  and  the  number  ( accurate  to 
five  places  of  decimals)  by  which,  if  the  radius  of  any 
circle  be  multiplied,  the  circumference  is  obtained. 


OF  DAY  AND  NIGHT.  103 
T  Which  now  is  the  more  probable  conclusion, 
either  that  the  earth  should  have  a  diurnal  motion  on 
its  axis  of  1000  miles  in  an  hour,  or  that  the  sun, 
which  is  a  million  of  times  larger  than  the  earth, 
should  travel  24  millions  of  miles  in  the  same  time  . 

J.  It  is  certainly  more  rational  to  conclude  that 
the  earth  turns  on  its  axis,  the  effect  of  which  you 
told  us  was  the  alternate  succession  of  day  and  mght. 

T.  I  did  ;  and  on  this  and  some  other  topics  we 
will  enlarge  to-morrow. 


CONVEKSATION  VIII. 

OF  DAY  AND  NIGHT. 

J.  You  are  now,  sir,  to  apply  the  rotation  of  the 
earth  about  its  axis  to  the  succession  of  day  and  night. 

r.  I  will ;  and  for  this  purpose  suppose  grcb  {tig. 
5.)  to  be  the  earth,  revolving  on  its  axis,  accordmg 
to  the  order  of  the  letters,  that  is,  from  gtor,  r  to  c, 
&c.  If  the  sun  be  fixed  in  the  heavens  at  s,  and  a 
line  ho  be  drawn  through  the  centre  of  the  earth  it 
will  represent  that  circle,  which,  when  extended  to 
the  heavens,  is  called  the  rational  horizon,  ^ 

C  In  what  does  this  differ  from  the  sensible  horizon  ? 

T  The  sensible  horizon  is  that  circle  m  the  heavens 
which  bounds  the  spectator's  view,  arid  which  is 
greater  or  less  according  as  he  stands  higher  or  lower, 
lor  example;  an  eye  placed  at  >e  feet  above  the 
surface  of  the  earth  or  sea,  sees  2-J  miles  every  way  : 
but  if  it  be  at  20  feet  high,  that  is,  four  times  the 
height,  it  will  see  5|  miles,  or  twice  the  distance. 

C  Then  the  senshle  differs  from  the  ratimial  hori- 
zon in  this,  that  the  former  is  seen  from  the  surface  ot 
the  earth,  and  the  latter  is  supposed  to  be  viewed 
from  its  centre.  .  .       „  , 

T.  You  are  right ;  and  the  rising  and  setting  ot  the 
sun  and  stars  are  always  referred  to  the  rational 
horizon.  .        ,  . 

/.  Why  sol  they  appear  to  rise  and  set  as  soon  as 


104  ASTRONOMY. 

they  get  above,  or  sink  below,  that  boundary  which 

separates  the  visible  from  the  invisible  part  of  the 

heavens. 

T,  They  do  not,  however ;  and  the  reason  is  this, 
that  the  distance  of  the  sun  and  fixed  stars  is  so  great 
m  comparison  of  4000  miles  (the  difference  between 
the  surface  and  centre  of  the  earth),  that  it  can 
scarcely  be  taken  into  account. 

C.  But  4000  miles  seem  to  me  an  imir  ense  space. 
T,  Considered  separately  they  are  so,  but  when 
compared  with  95  millions  of  miles,  the  distance  of 
the  sun  from  the  earth,  they  almost  vanish  as  nothing. 

J.  But  do  the  rising  and  setting  of  the  moon, 
which  is  at  the  distance  of  240  thousand  miles  onlv 
respect  also  the  rational  horizon  ?  " ' 

Certainly;  for  4000  compared  with  240,000, 
bear  only  the  proportion  of  1  to  60.  Now  if  two 
spaces  were  marked  out  on  the  earth  in  different 
directions,  the  one  60  and  the  other  61  yards,  should 
you  at  once  be  able  to  distinguish  the  greater  from 
the  less  \ 

C.  1  think  not. 

T.  Just  in  the  same  manner  does  the  distance  of 
the  centre  from  the  surface  of  the  earth  vanish  in 
comparison  of  its  distance  from  the  moon.  But  this 
IS  not  the  proper  time  to  explain  that  peculiar  differ- 
ence connected  with  what  astronomers  call  parallax, 

/.  We  must  not,  however,  forget  the  succession  of 
day  and  night. 

T.  Well  then  ;  if  the  sun  be  supposed  at  2,  it  will 
Illuminate  by  its  rays  all  that  part  of  the  earth  that  is 
above  the  horizon  ho;  to  the  inhabitants  at  its 
%yestern  boundary,  it  will  appear  just  rising ;  to  "those 
situated  at  r,  it  will  be  noon  ;  and  to  those  in  the 
eastern  part  of  the  horizon,  c,  it  will  be  setting. 

C.  Isee  clearly  why  it  should  be  noon  to  those 
who  live  at  r,  because  the  sun  is  just  over  their 
heads,  but  it  is  not  so  evident  why  the  sun  must  ap- 
pear rising  and  setting  to  those  who  are  at    and  c. 

i  .  \  ou  are  satisfied  that  a  spectator  cannot,  from 


OF  DAY  ANT)  NIGHT.  ^  105 

any  place,  observe  more  than  a  semi-circle  of  the 
heavens  at  any  one  time  ;  now  what  part  of  the  hea- 
vens will  the  spectator  at  g  observe  1 

J.  He  will  see  the  concave  hemisphere  zon, 
T.  The  boundary  to  his  view  will  be  s  and  n,  will 
it  not] 

C.  Yes  ;  and  consequently  the  sun  at  z  will  to 
him  be  just  coming  into  sight. 

T.  Then,  by  the  rotation  of  the  earth,  the  spectator 
at  g  will  in  a  few  hours  come  to  r,  when,  to  him,  it 
will  be  noon ;  and  those  who  live  at  r  will  have  de- 
scended to  c  ;  now  what  part  of  the  heavens  will  they 
see  in  this  situation  1 

J.  The  concave  hemisphere  nhz,  and  s  bemg  the 
boundary  of  their  view  one  way,  the  sun  will  to  them 
be  setting. 

T.  Just  so.  After  which  they  will  be  turned  away 
from  the  sun,,  and  consequently  it  will  be  night  to 
them  till  they  come  again  to  g.  Thus,  by  this  sim- 
ple motion  of  the  earth  on  its  axis,  every  part  of  it  is, 
by  turns,  enlightened  and  warmed  by  the  cheering 
beams  of  the  sun. 

C.  Does  this  motion  of  the  earth  account  also  for 
the  apparent  motion  of  the  fixed  stars  1 

r.  It  is  owing  to  the  revolution  of  the  earth  round 
its  axis,  that  we  imagine  the  whole  starry  firmament 
revolves  about  the  earth  in  24  hours. 

J.  If  the  heavens  appear  to  turn  on  an  axis,  must 
there  not  be  two  points,  namely,  the  extremities  of 
that  imaginary  axis,  which  always  keep  their  position  1 

T.  Yes,  we  must  be  understood  to  except  the  two 
celestial  poles  which  are  opposite  to  the  poles  of  the 
earth,  consequently  each  fixed  star  appears  to  de- 
scribe a  greater  or  a  less  circle  round  these,  accord- 
ing as  it°  is  more  or  less  remote  from  those  celestial 
poles.  .  , 

C.  When  we  turn  from  that  hemisphere  in  wnich 
the  sun  is  placed,  we  immediately  gain  sight  of  the 
other  in  which  the  stars  are  situated. 

F  2 


108 


ASTRONOMY. 


T,  Every  part  of  the  heavens  is  decorated  with 
these  glorious  bodies. 

/.  If  every  part  of  the  heavens  be  thus  adorned, 
v/hy  do  we  not  see  the  stars  in  the  day  as  well  as  the 
night '? 

T.  Because  in  the  day-time  the  sun's  rays  are  so 
powerful,  as  to  render  those  coming  from  the  fixed 
stars  invisible.  But  if  you  ever  happen  to  go  down 
into  any  very  deep  mine,  or  coal-pit,  where  the  rays 
of  the  sun  cannot  reach  the  eye,  and  it  be  a  clear  day, 
you  may,  by  looking  up  to  the  heavens,  see  the  stars 
at  noon  as  well  as  in  the  night. 

C.  If  the  earth  always  revolve  on  its  axis  in  24 
hours,  why  does  the  length  of  the  days  and  nights  dif- 
fer in  different  seasons  of  the  year  1 

T.  This  depends  on  other  causes  connected  with 
the  earth's  annual  journey  round  the  sun,  upon  which 
we  will  converse  the  next  time  we  meet. 


CONVERSATION  IX. 

OF  THE  ANNUAL  MOTION  OF  THE  EARTH. 

r.  Besides  the  diurnal  motion  of  the  earth,  by 
which  the  succession  of  day  and  night  is  produced, 
it  has  another,  called  its  annual  motion,  which  is  the 
journey  it  performs  round  the  sun  in  365  days,  5 
hours,  48  minutes,  and  49  seconds. 

C.  Are  the  different  seasons  to  be  accounted  for 
by  this  motion  of  the  earth  1 

T.  Yes,  it  is  the  cause  of  the  different  lengths  of 
the  days  and  nights,  and  consequently  of  the  different 
seasons,  viz.  Spring,  Summer,  Autumn,  and  Winter, 

J.  How  is  it  known  that  the  earth  makes  this  an- 
nual journey  round  tlie  sun  1 

T.  I  told  you  yesterday,  that  through  the  shaft  of 
a  very  deep  mine,  the  stars  are  visible  in  the  day  a3 
well  as  in  the  night ;  they  are  also  visible  in  the  day- 
time, by  means  of  a  telescope  properly  fitted  up  for 


ANNUAL  MOTION  OF  THE  EARTH.  107 

the  purpose ;  by  this  method,  the  sun  and  stars  are 
visible  at  the  same  time.  Now  if  the  sun  be  seen  in 
a  line  with  a  fixed  star  to-day  at  any  particular  hour, 
it  will,  in  a  few  weeks,  by  the  motion  of  the  earth, 
be  found  considerably  to  the  east  of  him :  and,  if  the 
observations  be  continued  through  the  year,  we  shall 
be  able  to  trace  him  round  the  heavens  to  the  same 
fixed  star  from  which  we  set  out ;  consequently,  the 
sun  must  have  made  a  journey  round  the  earth  in 
that  time,  or  the  earth  round  him. 

C.  And  the  sun  being  a  million  of  times  larger 
than  the  earth,  you  will  say  that  it  is  more  natural 
that  the  smaller  body  should  go  round  the  larger, 
than  the  reverse. 

T,  That  is  a  proper  argument ;  but  it  may  be  stated 
in  a  much  stronger  manner.  The  sun  and  earth  mu- 
tually attract  one  another,  and  since  they  are  in 
equilibrio  by  this  attraction,  you  know,  their  momenta 
must  be  equal;*  therefore  the  earth,  being  the  smaller 
body,  must  make  out  by  its  motion  what  it  wants  in 
the  quantity  of  its  matter,  and,  of  course,  it  must  be 
that  which  performs  the  journey. 

J.  But  if  you  refer  to  the  principle  of  the  lever,  to 
explain  the  mutual  attraction  of  the  sun  and  earth,  it 
is  evident,  that  both  bodies  must  turn  round  some 
point  as  a  common  centre. 

T,  They  do  ;  and  that  is  the  common  centre  of 
gravity  of  the  two  bodies.  Now  this  point  between 
the  earth  and  sun  is  within  the  surface  of  the  latter 
body. 

C.  I  understand  how  this  is  ;  because  tbe  centre 
of  gravity  between  any  two  bodies  must  be  as  much 
nearer  to  the  centre  of  the  larger  body  than  the 
smaller,  as  the  former  contains  a  greater  quantity  of 
matter  than  the  latter. 

T,  You  are  right :  but  you  will  not  conclude  that, 
because  the  sun  is  a  million  times  larger  than  the 
earth,  therefore  it  contains  a  quantity  of  matter  a 


*  See  Mechanics,  Conversation  XIV. 


108 


ASTRONOMY. 


million  of  times  greater  than  that  contained  in  the 
earth. 

J.  Is  it  then  known,  that  the  earth  is  composed  of 
matter  more  dense  than  that  which  composes  the  body 
of  the  sun  ? 

T,  The  earth  is  composed  of  matter  four  times 
denser  than  that  of  the  sun  ;  and  hence  the  quantity 
of  matter  in  the  sun  is  between  two  and  three  hun- 
dred thousand  times  greater  than  that  which  is  con- 
tained in  the  earth. 

C.  Then  for  the  momenta  of  these  two  bodies  to  be 
equal,  the  velocity  of  the  earth  must  be  between  two 
and  three  hundred  thousand  times  greater  than  that  of 
the  sun. 

T.  It  must :  and  to  effect  this,  the  centre  of  gravity 
between  the  sun  and  earth  must  be  as  much  nearer 
to  the  centre  of  the  sun,  than  it  is  to  the  centre  of  the 
earth,  as  the  former  body  contains  a  greater  quantity 
of  matter  than  the  latter  :  and  hence  it  is  found  to  be 
several  thousand  miles  within  the  surface  of  the  sun. 

J.  I  now  clearly  perceive,  that  since  one  of  these 
bodies  revolves  about  the  other  in  the  space  of  a  year, 
and  that  they  both  move  round  their  common  centre 
of  gravity,  that  it  must,  of  necessity,  be  the  earth 
which  revolves  about  the  sun,  and  not  the  sun  round 
the  earth. 

T.  Your  inference  is  just.  To  suppose  that  the 
sun  moves  round  the  earth,  is  as  absurd  as  to  main- 
tain, that  a  mill-stone  could  be  made  to  move  round  a 
pebble. 

CONVERSATION  X. 

OF  THE  SEASONS. 

T.  I  will  now  shew  you  how  the  different  seasons 
are  produced  by  the  annual  motion  of  the  earth. 

J.  Upon  what  do  they  depend,  sir  ? 

T.  The  variety  of  the  seasons  depends  (1)  upon 
tlie  length  of  the  days  and  nights  ;  and,  (2)  upon  the 
position  of  the  earth  with  respect  to  the  sun. 


OF  THE  SEASONS.  109 
C.  But,  if  the  earth  turn  round  its  imaginary  axis 
every  24  hours,  ought  it  not  to  enjoy  equal  days  and 
nights  all  the  year  1 

T,  This  would  be  the  case  if 
the  axis  of  the  earth  ns  were 
perpendicular  to  a  line  ce  drawn 
through  the  centres  of  the  sun 
and  earth  ;  for  then  as  the  sun 
always  enlightens  one  half  of 
the  earth  by  its  rays,  and  as  it  is  day  at  any  given 
place  on  the  globe  so  long  as  that  place  continues  m 
the  enlightened  hemisphere,  every  part  except  the  two 
poles  must,  during  its  rotation  on  its  axis,  be  one  half 
of  its  time  in  the  light  and  the  other  half  in  darkness; 
or,  in  other  words,  the  days  and  nights  would  be 
equal  to  all  the  inhabitants  of  the  earth,  excepting  to 
those,  if  any,  who  live  at  the  poles. 

J,  Why  do  you  except  the  people  at  the  poles  7 
r.  Because  the  view  of  the  spectator  situated  at 
the  poles  n  and  s,  must  be  bounded  by  the  line  ce  ; 
consequently  to  him  the  sun  would  never  appear  to 
rise  or  set,  but  would  always  be  in  the  horizon. 

C.  If  the  earth  were  thus  situated,  would  the  rays 
of  the  sun  always  fall  vertically  to  the  same  part  of  it? 

r.  They  would  :  and  that  part  would  be'  eq  the 
equator ;  and,  as  we  shall  presently  shew,  the  heat 
excited  by  the  sun  being  greater  or  less  in  proportion 
as  its  rays  come  more  or  less  perpendicularly  upon 
any  body,  the  parts  of  the  earth  about  the  equator 
would  be  scorched  up,  while  those  between  40  and 
50  degrees  on  each  side  of  that  line  and  the  poles 
would  be  desolated  by  an  unceasing  winter. 
J.  In  what  manner  is  this  prevented  1 
T,  By  the  axis  of  the  earth  ns 
being  inclined  or  bent  about  23 
degrees  and  a  half  out  of  the 
perpendicular.   In  this  case  you 
observe,  that  all  the  parallel 
circles,  except  the  equator,  are  Iig- 
divided  into  two  unequal  parts, 


110 


ASTRONOMY. 


having  a  greater  or  less  portion  of  their  circumferences 
in  the  enlightened,  than  in  the  dark  hemisphere, 
according  to  their  situation  with  respect  to  n  the 
north,  or  s  the  south  pole. 

C.  At  what  season  of  the  year  is  the  earth  repre- 
sented in  this  figure  1 

T.  At  our  summer  season :  for  you  observe  that 
the  parallel  circles  in  the  northern  hemisphere  have 
their  greater  parts  enlightened  and  their  smaller  parts 
in  the  dark.  If  dl  represent  that  circle  of  latitude  on 
the  globe  in  which  Great  Britain  is  situated,  it  is 
evident  that  about  two-thirds  of  it  is  in  the  light,  and 
only  one-third  in  darkness. 

You  will  remember  that  parallels  of  latitude  are 
circles  on  the  surface  of  the  earth,  or  its  representative 
the  terrestrial  globe,  drawn  parallel  to  the  equator. 

J.  Is  that  the  reason  why  our  days  towards  the 
middle  of  June  are  16  hours  long,  and  the  nights  but 
eight  hours  1 

T,  It  is  :  and  if  you  look  to  the  parallel  next  be- 
yond that  marked  d  l,  you  will  see  a  still  greater  dis- 
proportion between  the  day  and  night,  and  the  parallel 
more  north  than  this  is  entirely  in  the  light. 

C.  Is  it  then  all  day  there  1 

T.  To  the  whole  space  between  that  and  the  pole 
it  is  continual  day  for  some  time,  the  duration  of 
which  is  in  proportion  to  its  vicinity  to  the  pole  ;  and 
at  the  pole  there  is  a  permanent  day-light  for  six 
months  together. 

J.  And  during  that  time  it  must,  I  suppose,  be  night 
to  the  people  who  live  at  the  south  pole  1 

T,  Yes,  the  figure  shews  thatthe  south  pole  is  in  dark- 
ness ;  and  you  may  observe,  that  to  the  inhabitants  liv- 
ing in  equal  parallels  of  latitude,  the  one  north,  and  the 
other  south,  the  length  of  the  days  to  the  one  will  be 
always  equal  to  the  length  of  the  nights  to  the  other. 

C.  What  then  shall  we  say  to  those  who  live  at 
the  equator,  and  consequently  have  no  latitude  1 

T,  To  them  the  days  and  nights  are  alwaijs  equal, 
and  of  course  twelve  hours  each  in  length ;  and  this  is 


OF  THE  SEASONS.  HI 
also  evident  from  the  figure,  for  in  every  position  of 
the  globe  one  half  of  the  equator  is  m  the  light  and 
the  other  half  in  darkness.  ^ 

J.  If,  then  the  length  of  the  days  is  the  cause  ot 
the  diflferent  seasons,  there  can  be  no  variety  m  this 
respect  to  those  who  live  at  the  equator  1  ^ 

T.  You  seem  to  forget  that  the  change  m  the 
seasons  depends  upon  the  position  of  the  earth  with 
respect  to  the  sun,  that  is,  upon  the  perpendicuLarity 
with  which  the  rays  of  light  fall  upon  any  particular 
part  of  the  earth  ;  as  well  as  upon  the  length  oi  the 

^^C.*  Does  this  make  any  material  difference  with 
regard  to  the  heat  of  the  sun  1 

T.  It  does  :  let  a  b  represent 
a  portion  of  the  earth's  surface, 
on  which  the  sun's  rays  fall  per- 
pendicularly ;  let  BC  represent  an 
equal  portion  on  which  they  fall 
obliquely,  or  aslant.    It  is  mani-  Yw,  8. 

fest  that  BC,  though  it  be  equal  to  ° 
AB  receives  but  half  the  light  and  heat  that  ab  does. 
Moreover,  by  the  sun's  rays  coming  more  perpen. 
dicularly,  they  come  with  greater  force,  as  well  as  m 
greater  numbers,  on  the  same  place. 


CONVEKSATION  XI. 

OF  THE  SEASONS. 

T.  If  you  now  take  a  view  of  the  earth  in  its  an- 
nual course  round  the  sun,  considering  its  axis  as 
inclined  23|  degrees  to  a  line  perpendicular  to  its 
orbit,  and  keeping,  through  its  whole  journey,  a  di- 
rection  parallel  to  itself,  you  will  find,  that  accordmg 
as  the  earth  is  in  different  parts  of  its  orbit,  the  rays  ot 
the  sun  are  presented  perpendicularly  to  the  equator, 
and  to  every  point  of  the  globe  withm  23^  degrees  of 
it  both  north  and  south. 


112 


ASTRONOMY. 


Fig.  9. 

This  figure  represents  the  earth  in  four  different 
parts  of  its  orbit,  or  as  it  is  situated  with  respect  to 
the  sun  in  the  months  of  March,  June,  September, 
and  December. 

C.  The  earth's  orbit  is  not  made  circular  in  the 
figure. 

T,  No  ;  but  the  orbit  itself  is  nearly  circular  :  you 
are  supposed  to  view  it  from  the  under  side,  and  there- 
fore, though  almost  a  circle,  it  appears  to  be  a  long 
ellipse.  All  circles  appear  elliptical  in  an  oblique 
view,  as  is  evident  by  looking  obliquely  at  the  rim  of 
a  bason,  at  some  distance  from  you.  For  the  true 
figure  of  a  circle  can  only  be  seen  when  the  eye  is 
directly  over  its  centre.  You  observe  that  the  sun  is 
not  in  the  centre. 

J.  I  do  ;  and  it  appears  nearer  to  the  earth  in  the 
winter  than  in  the  summer. 

T.  We  are  indeed  more  than  three  millions  of  miles 
nearer  to  the  sun  in  December  than  we  are  in  June. 

C.  Is  tliis  possible,  and  yet  our  winter  is  so  much 
colder  than  the  summer  ? 

T.  Notwithstanding  this,  it  is  a  well-known  fact ; 
for  it  is  ascertained  that  our  summer,  that  is,  the  time 
that  passes  between  the  vernal  and  autumnal  equi- 
noxes, is  nearly  eight  days  longer  than  our  winter,  or 
the  time  between  the  autumnal  and  vernal  equinoxes. 


OF  THE  SEASONS. 


113 


Consequently  the  motion  of  the  earth  is  slower  in  the 
former  case  than  in  the  latter,  and  therefore,  as  we 
shall  see,  it  must  be  at  a  greater  distance  from  the 
sun.  Again,  the  sun's  apparent  diameter  is  greater  in 
our  winter  than  in  summer,  but  the  apparent  diameter 
of  any  object  increases  in  proportion  as  our  distance 
from  the  object  is  diminished,  and  therefore  we  con- 
clude, that  we  are  nearer  the  sun  in  winter  than  in 
summer.  The  sun's  apparent  diameter  in  winter  is 
32'.  .47"  ;  in  summer,  31'.  .40". 

J.  But  if  the  earth  is  farther  from  the  sun  in  sum- 
mer than  in  winter,  why  are  our  winters  so  much 
colder  than  our  summers  ? 

r.  Because  first,  in  the  summer,  the  sun  rises  to  a 
much  greater  height  above  our  horizon,  and  therefore 
its  rays  coming  rnore  perpendicularly,  more  of  them, 
as  we  shewed  you  yesterday,  must  fail  upon  the  sur- 
face of  the  earth,  and  come  also  with  greater  force, 
which  is  the  principal  cause  of  our  greater  summer's 
I  heat.    Secondly,  in  the  summer,  the  days  are  very 
i  long,  and  the  nights  short ;  therefore  the  earth  and 
i  air  are  heated  by  the  sun  in  the  day  more  than  they 
I  are  cooled  in  the  night. 

I  J.  Why  have  we  not,  then,  the  greatest  heat  at  the 
time  when  the  days  are  longest  1 

'  T.  The  hottest  season  of  the  year  is  certainly  a 
month  or  two  after  this,  which  may  be  thus  accounted 
for.    A  body  once  heated  does  not  grow  cold  again 

i  instantaneously,  but  gradually  :  now,  as  long  as  more 
heat  comes  from  the  sun  in  the  day  than  is  lost  in 
the  night,  the  heat  of  the  earth  and  air  will  be  daily 
increasing,  and  this  must  evidently  be  the  case  for 
some  weeks  after  the  longest  day,  both  on  account  of 
the  number  of  rays  which  fall  on  a  given  space,  and 
also  from  the  perpendicular  direction  of  those  rays. 

J,  Will  you  now  explain  to  us  in  what  manner  the 
seasons  are  produced  1 

T,  By  referring  to  the  last  figure  you  will  observe, 

'   that  in  the  month  of  June  the  north  pole  of  the  earth 

;   inclines  towards  the  sun,  and  consequently  brings  ail 

I 

i 


114 


ASTRONOMY. 


the  northern  parts  of  the  globe  more  into  light,  than 
at  any  other  time  in  the  year. 

C  Then  to  the  people  in  those  parts  it  is  summer  ? 

r.  It  is :  but  in  December,  when  the  earth  is  in 
the  opposite  part  of  its  orbit,  the  north  pole  declines 
from  the  sun,  which  occasions  the  northern  places 
to  be  more  in  the  dark  than  in  the  light  3  and  the  re- 
verse at  the  southern  places. 

J.  Is  it  then  summer  to  the  inhabitants  of  the 
southern  hemisphere  ? 

T,  Yes,  it  is  ;  and  winter  to  us.  In  the  months  of 
March  and  September  the  axis  of  the  earth  does  not 
incline  to,  nor  decline  from,  the  sun,  but  is  perpen- 
dicular to  a  line  drawn  from  its  centre.  And  then 
the  poles  are  in  the  boundary  of  light  and  darkness, 
and  the  sun  being  directly  vertical  to,  or  over,  the 
equator,  makes  equal  day  and  night  at  all  places. 
jNow  trace  the  annual  motion  of  the  earth  in  its  orbit 
for  yourself,  as  it  is  represented  in  the  figure. 

C.  I  will,  sir  :  about  the  20th  of  March  the  earth 
is  in  Libra,  and  consequently  to  its  inhabitants  the 
sun  will  appear  in  Aries,  and  be  vertical  to  the 
equator. 

T.  Then  the  equator  and  all  its  parallels  are 
equally  divided  between  the  light  and  dark. 

C.  Consequently,  the  days  and  nights  are  equal  all 
over  the  world.  As  the  earth  pursues  its  journey 
from  March  to  June  its  northern  hemisphere  comes 
more  into  light,  and  on  the  21st  of  that  month  tlie 
sun  is  vertical  to  the  tropic  of  Cancer. 

T.  You  then  observe,  that  all  the  circles  parallel 
to  the  equator  are  unequally  divided;  those  in  the 
northern  half  have  their  greater  parts  in  the  light,  and 
those  in  the  southern  half  have  their  larger  parts  in 
darkness. 

C.  Yes  ;  and,  of  course,  it  is  summer  to  the  in- 
habitants of  the  northern  hemisphere,  and  winter  to 
the  southern. 

I  now  trace  it  to  September,  when  I  find  the  sun 
vertical  again  to  the  equator,  and,  of  course,  the  days 


OF  THE  SEASONS. 


115 


and  nights  are  again  equal.  And  following  the  earth 
in  its  journey  to  December,  or  when  it  has  arrived  at 
Cancer,  the  sun  appears  in  Capricorn,  and  is  vertical 
to  that  part  of  the  earth  called  the  tropic  of  Capri- 
corn; and  now  the  southern  pole  is  enlightened,  and 
all  the  circles  on  that  hemisphere  have  their  larger 
parts  in  light,  and,  of  course,  it  is  summer  to  those 
parts,  and  winter  to  us  in  the  northern  hemisphere. 

r.  Can  you,  James,  now  tell  me,  why  the  days 
lengthen  and  shorten  frorn  the  equator  to  the  polar 
circles  every  year  1 

J.  I  will  try  to  explain  myself  on  the  subject. 
I  Because  the  sun  in  March  is  vertical  to  the  equator, 
'  and  from  that  time  to  the  21st  of  June  it  becomes 
I  vertical  successively  to  all  other  parts  of  the  earth  be- 
j  tween  the  equator  and  the  tropic  of  Cancer  ;  and  in 
I  proportion  as  it  becomes  vertical  to  the  more  northern 
I  parts  of  the  earth,  it  declines  from  the  southern,  and, 
I  consequently,  to  the  former  the  days  lengthen,  and  to 
!  the  latter  they  shorten.  From  June  to  September  the 
!  sun  is  again  vertical  successively  to  all  the  same  parts 
I   of  the  earth,  but  in  a  reverse  order. 

C.  Since  it  is  summer  to  all  those  parts  of  the 
1   earth  where  the  sun  is  vertical,  and  we  find  that  the 
j   sun  is  vertical  twice  in  the  year  to  the  equator,  and 
every  part  of  the  globe  between  the  equator  and 
tropics,  there  must  be  also  two  summers  in  a  year  to 
all  those  places. 

r.  There  are  ;  and  in  those  parts  near  the  equator 
they  have  two  harvests  every  year. — But  let  your 
brother  finish  his  description. 

J.  From  September  to  December  it  is  successively 
vertical  to  all  the  parts  of  the  earth  situated  between 
the  equator  and  the  tropic  of  Capricorn,  which  is  also 
the  cause  of  the  lengthening  of  the  days  in  the 
southern  hemisphere,  and  of  their  becoming  shorter  in 
the  northern. 

I        T.  Can  you,  Charles,  tell  me  why  there  is  some- 
'    times  no  day  or  night  for  some  little  time  together 
within  the  polar  circles  1 

\ 


116 


ASTRONOMY. 


C.  The  sun  always  shines  upon  the  earth  90  de- 
grees every  way,  and  when  he  is  vertical  to  the  tropic 
of  Cancer,  which  is  23|  degrees  north  of  the  equator, 
he  must  shine  the  same  number  of  degrees  beyond  the 
pole,  or  to  the  polar  circle,  and  while  he  thus  shines 
there  can  be  no  night  to  the  people  within  that  polar 
circle,  and,  of  course,  to  the  inhabitants  at  the  south- 
ern polar  circle  there  can  be  no  day  at  the  same 
time  ;  for  as  the  sun's  rays  reach  but  90  degi'ees  every 
way,  they  cannot  shine  far  enough  to  reach  them. 

T,  Tell  me,  now,  why  there  is  but  one  day  and 
night  in  the  whole  year  at  the  poles  1 

C.  For  the  reason  which  I  have  just  given,  the  sun 
must  shine  beyond  the  north  pole  all  the  time  he  is 
vertical  to  those  parts  of  the  earth  situated  between 
the  equator  and  the  tropic  of  Cancer,  that  is,  from 
March  the  21st  to  September  the  20th,  during  which 
time  there  can  be  no  night  at  the  north  pole,  nor  any 
day  at  the  south  pole.  The  reverse  of  this  may  be 
applied  to  the  southern  pole. 

J.  I  understand  now,  that  the  lengthening  and 
shortening  of  the  days,  and  different  seasons,  are  pro- 
duced by  the  annual  motion  of  the  earth  round  the 
sun  ;  the  axis  of  the  earth,  in  all  parts  of  its  orbit, 
being  kept  parallel  to  itself.  But,  if  thus  parallel  to 
itself,  hov/  can  it  in  all  positions  point  to  the  pole- 
star  in  the  heavens  ? 

T,  Because  the  diameter  of  the  earth's  orbit  is 
nothing  in  comparison  of  the  distance  of  the  earth 
from  the  fixed  stars.  Suppose  you  draw  two  parallel 
lines  at  the  distance  of  three  or  four  yards  from  one 
another,  will  they  not  both  point  to  the  moon  when 
she  is  in  the  horizon  ? 

/.  Three  or  four  yards  cannot  be  accounted  as  any 
thing  in  comparison  of  240  thousand  miles,  the  dis- 
tance of  the  moon  from  us. 

T.  Perhaps  three  yards  bear  a  greater  proportion 
to  240  thousand  miles,  than  190  millions  of  miles 
bear  to  our  distance  from  the  polar  star. 


OF  THE  EQUATION  OF  TIME. 


117 


CONVERSATION  XII. 

OF  THE   EQUATION  OF  TIME. 

T.  You  are  now,  I  presume,  acquainted  with  the 
motions  peculiar  to  this  globe  on  which  we  live '? 

C.  Yes  :  it  has  a  rotation  on  its  axis  from  west  to 
east  every  24  hours,  by  which  day  and  night  are  pro- 
duced, and  also  the  apparent,  diurnal  motion  of  the 
heavens  from  east  to  west. 

J.  The  other  is  its  annual  revolution  m  an  orbit 
round  the  sun,  likewise  from  west  to  east,  at  the  dis- 
tance of  about  95  millions  of  miles  from  the  sun. 

T.  You  understand,  also,  in  what  manner  this  an- 
nual motion  of  the  earth,  combined  with  the  inclina- 
tion of  its  axis,  is  the  cause  of  the  variety  of  seasons. 

We  will  therefore  proceed  to  investigate  another 
curious  subject,  viz.  the  equation  of  time,  and  to  ex- 
plain to  you  the  difterence  between  equal,  or  mean, 
and  apparent  time. 

C.  Will  you  tell  us  what  you  mean  by  the  words 
equal  and  apparent,  as  applied  to  time 

r.  Equal  or  mean  time  is  measured  by  a  clock, 
that  is  supposed  to  go  without  any  variation,  and  to 
measure  exactly  24  hours  from  noon  to  noon.  And 
apparent  time  is  measured  by  the  apparent  motion  ot 
the  sun  in  the  heavens,  or  by  a  good  sun-dial. 

C.  And  what  do  you  mean,  sir,  by  the  equation  oj 
time?  . 

T,  It  is  the  adjustment  of  the  difference  ot  time, 
as  shewn  by  a  well-regulated  clock  and  a  true  sun- 
dial. 

J.  Upon  what  does  this  difference  depend  1 
T.  It  depends,  first,  upon  the  inclination  of  the 
earth's  axis  ;  and,  secondly,  upon  the  elliptic  form  of 
the  earth's  orbit ;  for,  as  we  have  already  seen,  the 
earth's  orbit  being  an  ellipse,  its  motion  is  quicker 
when  it  is  in  perihelion,  or  nearest  to  the  sun  ;  and 
slower  when  it  is  in  aphelion,  or  farthest  from  the  sun. 


118 


ASTRONOMY. 


C.  But.  I  do  not  yet  comprehend  what  the  rotation 
of  the  earth  has  to  do  with  the  going  of  a  clock  or 
watch. 

T.  The  rotation  of  the  earth  is  the  most  equable 
and  uniform  motion  in  nature,  and  is  completed  in 
23  hours,  56  minutes,  and  4  seconds ;  this  space  of 
time  is  called  a  sidereal  day,  because  any  meridian  on 
the  earth  will  revolve  from  a  fixed  star  to  that  star 
again  in  this  time.  But  a  solar,  or  natural  day,  which 
our  clocks  are  intended  to  measure,  is  the  tinie  which 
any  meridian  on  the  earth  will  take  in  revolving  from 
the  sun  to  the  sun  again,  which  is  about  24  hours, 
sometimes  a  little  more,  but  generally  less. 

J.  What  occasions  this  difference  between  the  so- 
lar and  sidereal  day  ? 

T,  The  distance  of  the  fixed  stars  is  so  great,  that 
the  diameter  of  the  earth's  orbit,  though  190  millions 
of  miles,  when  compared  with  it,  is  but  a  point,  and 
therefore  any  meridian  on  the  earth  will  revolve  from 
a  fixed  star  to  that  star  again  in  exactly  the  same 
time  as  if  the  earth  had  only  a  diurnal  motion,  and 
remained  always  in  the  same  part  of  its  orbit.  But 
with  respect  to  the  sun,  as  the  earth  advances  almost 
a  degree  eastward  in  its  orbit,  in  the  same  time  that 
it  turns  eastward  round  its  axis,  it  must  make  more 
than  a  complete  rotation  before  it  can  come  into  the 
same  position  with  the  sun  that  it  had  the  day  before. 
In  the  same  way  as  when  both  the  hands  of  a  watch 
or  clock  set  off  together  at  twelve  o'clock,  the  minute- 
hand  must  travel  m.ore  than  a  whole  circle  before  it 
will  overtake  the  hour-hand,  that  is,  before  they  will 
be  in  the  same  relative  position  again.  Thus  the 
sidereal  days  are  shorter  than  the  solar  ones  by  about 
four  minutes,  as  is  evident  from  observation. 

C.  Still  I  do  not  understand  the  reason  why  the 
clocks  and  dials  do  not  agree. 

T.  A  good  clock  is  intended  to  measure  that  equa- 
ble and  uniform  time  which  the  rotation  of  the  earth 
on  its  axis  exhibits  ;  whereas  the  dial  measures  time 
by  the  apparent  motion  of  the  sun,  which,  as  we  have 


OF  THE  EQUATION  OF  TIME.  119 
explained,  is  subject  to  variation.  Or  thus  :  though 
the  earth's  motion  on  its  axis  be  perfectly  uniform, 
and  consequently  the  rotation  of  the  equator,  the 
plane  of  which  is  perpendicular  to  the  axis,  or  of  any 
other  circle  parallel  to  it,  be  likewise  equable,  yet  we 
measure  the  length  of  the  natural  day  by  means  of 
the  sun,  whose  apparent  annual  motion  is  not  in  the 
equator,  or  any  of  its  parallels,  but  in  the  ecliptic, 
which  is  oblique  to  it. 

/.  Do  you  mean  by  this  that  the  equator  of  the 
earth,  in  its  annual  journey,  is  not  always  directed 
towards  the  centre  of  the  sun  ? 

T.  I  do ;  twice  only  in  the  year,  a  line  drawn  from 
the  centre  of  the  sun  to  that  of  the  earth  passes 
through  those  points  where  the  equator  and  ecliptic 
cross  one  another  ;  at  all  other  times,  it  passes  through 
some  other  part  of  that  oblique  circle  which  is  repre- 
sented on  the  globe  by  the  ecliptic  line.  Now  when 
it  passes  through  the  equator,  or  the  tropics,  which 
are  circles  parallel  to  the  equator,  the  sun  and  clocks 
go  together,  as  far  as  regards  this  cause,  but  at  other 
times  they  differ,  because  equal  portions  of  the  eclip- 
tic pass  over  the  meridian  in  unequal  parts  of  time, 
on  account  of  its  obliquity. 

C.  Can  you  explain  this  by  a  figure  1 

T.  It  is  easily  shewn  by  the  n 
globe,  which  this  figure  ^  n 
=?!b  s  may  represent :  cy'  =^ 
will  be  the  equator,  <y>  53  — 
the  northern  half  of  the  eclip-  t 
tic,  and  c\pVS  ~  the  southern 
half.  Make  chalk  or  pencil 
marks  a,  h,  c,  d,  e,f,  h,  all 
round  the  equator  and  ecliptic, 
at  equal  distances  (suppose  20 
degrees)  from  each  other,  be- 
ginning at  Aries.  Now  by  turning  the  globe  on  its 
axis,  you  will  perceive  that  all  the  marks  in  the  first 
quadrant  of  the  ecliptic,  that  is,  from  Aries  to  Cancer, 
come  sooner  to  the  brazen  meridian  than  their  corre- 


120 


ASTRONOMY. 


spending  marks  on  the  equator: — those  from  the  be- 
ginning of  Cancer  to  Libra  come  later: — those  from 
Libra  to  Capricorn  sooner : — and  those  from  Capri- 
corn to  Aries  later. 

Now  time  as  measured  by  the  sun-dial  is  repre- 
sented by  the  marks  on  the  ecliptic;  that  measured 
by  a  good  clock,  by  those  on  the  equator, 

C.  Then  while  the  sun  is  in  the  first  and  third 
quarters,  or,  what  is  the  same  thing,  while  the  earth  is 
travelling  through  the  second  and  fourth  quarters, 
that  is,  from  Cancer  to  Libra,  and  from  Capricorn  to 
Aries,  the  sun  is  faster  than  the  clocks,  and  while  it 
is  travelling  the  other  two  quarters  it  is  slower. 

T.  Just  so :  because  while  the  earth  is  travelling 
through  the  second  and  fourth  quadrants,  equal  por- 
tions of  the  ecliptic  come  sooner  to  the  meridian  than 
their  corresponding  parts  of  the  equator ;  and  during 
its  journey  through  the  first  and  third  quadrants,  the 
equal  parts  of  the  ecliptic  arrive  later  at  the  meridian 
than  their  corresponding  parts  of  the  equator. 

J.  If  I  understand  what  you  have  been  saying,  the 
dial  and  clocks  ought  to  agree  at  the  equinoxes,  that 
'is,  on  the  20th  of  March  and  the  23d  of  September ; 
but  if  I  refer  to  the  Ephemeris,  I  find  that  on  the  for- 
mer day  (1822)  the  clock  is  nearly  eight  minutes 
before  the  sun ;  and  on  the  latter  day  the  clock  is 
more  than  seven  minutes  behind  the  sun. 

T.  If  this  difference  between  time  measured  by  the 
dial  and  clock  depended  only  on  the  inclination  of 
the  earth's  axis  to  the  plane  of  its  orbit,  the  clocks 
and  dial  ought  to  be  together  at  the  equinoxes,  and 
also  on  the  21st  of  June  and  the  21st  of  December, 
that  is,  at  the  summer  and  winter  solstices ;  because, 
on  those  days,  the  apparent  revolution  of  the  sun  is 
parallel  to  the  equator.  But  I  told  you  there  was 
another  cause  why  this  difference  subsisted. 

C.  You  did  ;  and  that  was  the  elliptic  form  of  the 
earth's  orbit. 

T.  If  the  earth's  motion  in  its  orbit  were  uniform, 
which  it  would  be  if  the  orbit  were  circular,  then  the 


OF  LEAP  YEAR. 


121 


whole  difference  between  equal  time  as  shewn  by 
the  clock,  and  apparent  time  as  shewn  by  the  sun, 
would  arise  from  the  inclination  of  the  earth's  axis. 
But  this  is  not  the  case  ;  for  the  earth  travels,  when 
it  is  nearest  the  sun,  that  is,  in  the  winter,  more  than 
a  degree  in  24  hours,  and  when  it  is  farthest  from  the 
sun,  that  is,  in  summer,  less  than  a  degree  in  the 
same  time  ;  consequently,  from  this  cause,  the  natu- 
ral day  would  be  of  the  greatest  length  when  the 
earth  was  nearest  the  sun,  for  it  must  continue  turn- 
ing the  longest  time  after  an  entire  rotation,  in  order 
to  bring  the  meridian  of  any  place  to  the  sun  again  ; 
and  the  shortest  day  would  be  when  the  earth  moves 
the  slowest  in  her  orbit.  Now  these  inequalities, 
combined  with  those  arising  from  the  inclination  of 
the  earth's  axis,  make  up  that  difference  which  is 
shewn  by  the  equation  table,  found  in  the  Epheme- 
ris,  between  good  clocks  and  true  sun-dials. 


CONVERSATION  XIIE 

OF  LEAP-YEAR. 

J.  Before  we  quit  the  subject  of  time,  will  you 
give  us  some  account  of  what  is  called  in  our  alma- 
nacs Leap- Year  ? 

T.  I  will.  The  length  of  our  year  is,  as  you  know, 
measured  by  the  time  which  the  earth  takes  in  per- 
forming her  journey  round  the  sun,  in  the  same  man- 
ner as  the  length  of  the  day  is  measured  by  its  rota- 
tion on  its  axis.  Now,  to  compute  the  exact  time 
taken  by  the  earth  in  its  annual  journey,  was  a  work 
of  considerable  difficulty.  Julius  Caesar  was  the  first 
person  who  seems  to  have  attained  to  any  accuracy 
on  this  subject. 

C.  Do  3^ou  mean  the  first  Roman  Emperor,  who 
landed  also  in  Great  Britain  ? 

T.  I  do.»  He  was  not  less  celebrated  as  a  man  of 
science,  than  he  was  renowned  as  a  general,  Julius 
Caesar,  who  was  well  acquainted  with  the  learning  of 
G 


122 


ASTRONOMY. 


the  Egyptians,  fixed  the  length  of  the  year  to  be  365 
days  and  six  hours,  which  made  it  six  hours  longer 
than  the  Egyptian  year.  Now,  in  order  to  allow  for 
the  odd  six  hours  in  each  year,  he  introduced  an  ad- 
ditional day  every  fourth  year,  which  accordingly 
consists  of  366  days,  and  is  called  Lea;^- Year,  while 
the  other  three  have  only  365  days  each.  From  him 
it  was  denominated  the  Julian  year. 

/.  It  is  also  called  Bissextile  in  the  almanacs ; 
what  does  that  mean  1 

T.  The  Romans  inserted  the  intercalary  day  be- 
tween the  23d  and  24tli  of  February ;  and  because 
the  23d  of  February,  in  their  calendar,  was  called 
sexto  calendas  Martii,  the  6th  of  the  calends  of  March, 
the  intercalated  day  was  called  his  sexto  calendas  Mar- 
tii,  the  second  sixth  of  the  calends  of  March,  and 
hence  the  year  of  intercalation  had  the  appellation 
of  Bissextile.  This  day  was  chosen  at  Rome,  on 
account  of  the  expulsion  of  Tarquin  from  the  throne, 
which  happened  on  the  23d  of  February.  We  intro- 
duce, in  Leap- Year,  a  new  day  in  the  same  month 
namely,  the  29th. 

C.  Is  there  any  rule  for  knowing  what  year  is 
Leap- Year  1 

T,  It  is  known  by  dividing  the  date  of  the  year  by 
4;  if  there  be  no  remainder  it  is  Leap- Year  ;  thus 
1831  divided  by  4  leaves  a  remainder  of  3,  shewing 
that  it  is  the  third  year  after  Leap- Year.  These  two 
lines  contain  the  rule  : 

Divide  by  4;  what's  left  shall  be 
For  Leap-year  0 ;  for  past  1,  2,  3. 

J.  The  year,  however,  does  not  consist  of  365 
days  and  6  hours,  but  of  365  days,  5  hours,  48  mi- 
nutes, and  49  seconds.*  Will  not  this  occasion  some 
error  1 

T.  It  will  ;  and,  by  subtracting  the  latter  number 
from  the  former,  you  will  find  that  the  error  amounts 


*  See  Conversation  IX. 


OF  LEAP  YEAR. 


123 


to  11  minutes  and  11  seconds  every  year,  or  to  a 
whole  day  in  about  130  years  :  notwithstanding  this, 
the  Julian  year  continued  to  be  in  general  use  till  the 
year  1582,  when  Pope  Gregory  the  13th  undertook 
to  rectify  the  error,  which  at  that  time  amounted  to 
10  days.     He  accordingly  commanded  the  10  days 
between  the  4th  and  15th  of  October  in  that  year  to 
be  suppressed,  so  that  the  5th  day  of  that  month  was 
called  the  15th.    This  alteration  took  place  through 
I  the  greater  part  of  Europe,  and  the  year  was  after- 
ii  wards  called  the  Gregorian  year,  or  New  Style,  In 
li  this  country,  the  method  of  reckoning  according  to 
the  New  Style  was  not  admitted  into  our  calendars 
{  till  the  year  1752,  when  the  error  amounted  to  nearly 
j  11  days,  which  were  taken  from  the  month  of  Sep- 
tember,  by  calling  the  3d  of  that  month  the  14th. 

C.  By  what  means  will  this  accuracy  be  main- 
tained? 

;  T,  The  error  amounting  to  one  whole  day  in  about 
:  130  years,  it  is  settled  by  an  act  of  parliament,  that 
I  the  year  1800  and  the  year  1900,  which  are,  accord- 
ing to  the  rule  just  given,  Leap-years,  shall  be  com- 
i  puted  as  common  years,  having  only  365  days  in 
;  each ;  and  that  every  four  hundredth  year  afterwards 
ij  shall  be  common  years  also.  If  this  method  be  ad- 
I  hered  to,  the  present  mode  of  reckoning  will  not  vjiry 

a  single  day  from  true  time  in  less  than  5000  years. 
\     By  the  same  act  of  parliament,  the  legal  beginning 
!'  of  the  year  was  changed  from  the  25th  of  March  to 
the  1st  of  January.    So  that  the  succeeding  months 
of  January,  February,  and  March,  up  to  the  24th 
day,  which  would,  by  the  Old  Style,  have  been 
i  reckoned  part  of  the  year  1752,  were  accounted  as 
'  the  three  first  months  of  the  year  1753.    Which  is 
the  reason  you  sometimes  meet  with  such  a  date  as 
this,  March  15,  1774-5  ;  that  is,  according  to  the 
Old  Style  it  was   1774 — according  to  the  New, 
1775.    Russia  is  the  only  country  in  Europe  where 
the  Old  Style  still  prevails. 


124 


ASTRONOMY. 


CONVERSATION  XIV 

OF  THE  MOON. 

T,  You  are  now,  gentlemen,  acquainted  with  the 
reasons  for  the  division  of  time  into  days  and  years. 

C.  These  divisions  have  their  foundation  in  nature, 
former  depending  upon  the  rotation  of  the  earth 
on  its  axis ;  the  lalter  upon  its  revolution  in  an  ellip- 
tic orbit  about  the  sun  as  a  centre  of  motion. 

J.  Is  there  any  natural  reason  for  the  division  of 
years  into  weeks,  or  of  days  into  hours,  minutes,  and 
seconds ] 

T.  The  first  of  these  divisions  was  introduced  by 
Divine  Authority ;  the  second  class  was  invented  for 
the  convenience  of  mankind.  There  is,  however, 
another  division  of  time  marked  out  by  nature. 

C.  What  is  that,  sir? 

T.  The  length  of  the  month:  not,  indeed,  that 
month  which  consists  of  four  weeks,  nor  that  by 
which  the  year  is  divided  into  12  parts.  These  are 
both  arbitrary.  But  by  a  month  is  meant  the  time 
which  the  moon  takes'  in  performing  her  journey 
round  the  earth. 

J.  How  many  days  does  t^e  moon  take  for  this 
purpose  1 

T.  If  you  refer  to  the  time  in  which  the  m.oon  re- 
volves from  one  point  of  the  heavens  to  the  same 
point  again,  it  consists  of  27  days,  7  hours,  and  43 
minutes  ;  this  is  called  the  periodical  month  :  but  if 
you  refer  to  the  time  passed  from  new  moon  to  new 
moon  again,  the  month  consists  of  29  days,  12  hours, 
and  44  minutes  ;  this  is  called  the  siinociical  month. 

C,  Pray  explain  the  reason  of  this  difference. 

r.  It  is  occasioned  by  the  earth's  annual  motion 
in  its  orbit.  Let  us  refer  to  our  watch  as  an  example. 
The  two  hands  are  together  at  12  o'clock ;  now,  when 
the  minute-hand  has  made  a  complete  revolution,  are 
they  together  again  1: 


OP  THE  MOON.  125 

J.  No  ;  for  the  hour-hand  is  advanced  the  twelfth 
part  of  its  revolution,  which,  in  order  that  the  other 
may  overtake,  it  must  travel  five  minutes  more  than 
the  hour. 

T.  And  something  more,  for  the  hour-hand  does 
not  wait  at  the  figure  1,  till  the  other  comes  up  ;  and 
therefore  they  will  not  be  together  till  between  five 
and  six  minutes  after  one. 

Now  apply  this  to  the  earth  and  moon )  suppose 


Fig.  11. 


3  to  be  the  sun ;  t  the  earth,  in  a  part  of  its  orbit  ql  ; 
and  E  to  be  the  position  of  the  moon  :  if  the  earth 
had  no  motion,  the  moon  would  miOve  round  its 
orbit  EHC  into  the  position  e  again  in  27  days,  7 
hours,  43  minutes ;  but  while  the  moon  is  describing 
her  journey,  the  earth  has  passed  through  nearly  a 
twelfth  part  of  its  orbit,  which  the  moon  must  also 
describe  before  the  two  bodies  come  again  into  the 
same  position  that  they  before  held  with  respect  to 
the  sun  :  this  takes  up  so  much  more  time  as  to  make 
her  synodical  month  equal  to  29  days,  12  hours,  and 
44  minutes :  hence  the  foundation  of  the  division  of 
time  into  months. 

We  will  now  proceed  to  describe  some  other  parti- 
culars relating  to  the  moon,  as  a  body  depending, 
like  the  earth,  on  the  sun  for  her  light  and  heat. 

C.  Does  the  moon  shine  with  a  borrowed  lio-ht 
only  ? 


120  ASTRONOMY. 

7'.  This  is  certain  j  for  if,  like  the  sun,  she  were  a 
luminous  body,  she  would  always  shine  with  a  full 
orb,  as  the  sun  does.  Her  diameter  is  nearly  2200 
miles  in  length. 

J.  And  1  remember  she  is  at  the  distance  of 
240,000  miles  from  the  earth. 

7\  The  sun  s  (Fig.  11.)  always  enlightens  one 
half  of  the  moon  e,  and  its  whole  enlightened  hemi- 
sphere, or  a  part  of  it,  or  none  at  all,  is  seen  by  us 
according  to  her  different  positions  in  the  orbit  with 
respect  to  the  earth  ;  for  only  those  parts  of  the  en- 
lightened half  of  the  moon  are  visible  at  t  which  are 
cut  off  by,  and  are  within,  the  orbit. 

J,  Then  when  the  moon  is  at  e,  no  part  of  its  en- 
lightened side  is  visible  to  the  earth. 

T.  You  are  right :  it  is  then  new  moon,  or  change, . 
for  it  is  usual  to  call  it  new  moon  the  first  day  it  is 
visible  to  the  earth,  which  is  not  till  the  second  day 
after  the  change.  And  the  moon  being  in  a  line  be- 
tween the  sun  and  earth,  they  are  said  to  be  in  con- 
junction, 

C.  And  at  a  all  the  illuminated  hemisphere  is 
turned  to  the  earth. 

T.  This  is  called  full  moon;  and  the  earth  being 
between  the  sun  and  moon,  they  are  said  to  be  in 
opposition.  The  enlightened  parts  of  the  little  figures 
on  the  outside  of  the  orbit,  represent  the  appearance 
of  the  moon  as  seen  by  a  spectator  on  the  earth. 

J.  Is  the  little  figure  then  opposite  e  wholly  dark 
to  shew  that  the  moon  is  invisible  at  change  ? 

2\  It  is :  and  when  it  is  at  f  a  small  part  of  the 
illuminated  hemisphere  is  icithin  the  moon's  orbit,  and 
therefore  to  a  spectator  at  t  it  appears /lorned;  at  g 
one  half  of  the  enlightened  hemisphere  is  visible,  and 
it  is  said  to  be  in  quadrature :  at  h  three-fourths  of 
the  enlightened  part  is  visible  to  the  earth,  and  it  is 
then  said  to  be  gibbous :  and  at  a  the  whole  enlight- 
ened face  of  the  moon  is  turned  to  the  earth,  and  it  is 
said  to  he  fall.    The  same  may  be  said  of  the  rest. 

The  horns  of  the  moon,  before  conjunction  or  new 


OF  THE  MOON. 


127 


moon,  are  turned  to  the  cast:  after  conjunction  they 
are  turned  to  the  west. 

C.  I  see  the  figure  is  intended  to  shew  that  the 
moon's  orbit  is  elliptical :  does  she  also  turn  upon  her 
axis  1 

T.  She  does  ;  and  she  requires  the  same  time  for 
her  diurnal  rotation  as  she  takes  in  completing  her 
revolution  about  the  earth  :  and  consequently,  though 
every  part  of  the  moon  is  successively  presented  to 
the  sun,  yet  the  same  hemisphere  is  alv^^ays  turned  to 
the  earth.  This  is  knov^^n  by  observation  with  good 
telescopes. 

/.  Then  the  length  of  a  day  and  night  in  the  moon 
is  equal  to  more  than  twenty-nine  days  and  a  half  of 
ours. 

r.  It  is  so  :  and  therefore,  as  the  length  of  her 
year,  which  is  measured  by  her  journey  round  the 
sun,  is  equal  to  that  of  ours,  she  can  have  but  about 
twelve  days  and  one-third  in  a  year.  Another  re- 
markable circumstance  relating  to  the  moon  is,  that 
the  hemisphere  next  the  earth  is  never  in  darkness  ; 
for  in  the  position  e,  when  it  is  turned  from  the  sun, 
it  is  illuminated  by  light  reflected  from  the  earth,  in 
the  same  manner  as  we  are  enlightened  by  a  full 
moon.  But  the  other  hemisphere  of  the  moon  has  a 
fortnight's  light  and  darkness  by  turns. 

C.  Can  the  earth,  then,  be  considered  as  a  satellite 
to  the  moon  1 

r.  It  would,  perhaps,  be  maccurate  to  denominate 
the  larger  body  a  satellite  to  the  smaller ;  but  with 
regard  to  affording  reflected  light,  the  earth  is  to  the 
moon  what  the  moon  is  to  the  earth,  and  subject  to 
the  same  changes  of  horned,  gibbous,  full,  &c. 

C.  But  it  must  appear  much  larger  than  the  moon. 

r.  The  earth  will  appear  to  the  inhabitants  of  the 
moon  about  13  times  as  large  as  the  moon  appears  to 
us.  When  it  is  new  moon  to  us  it  is  full  earth  to 
them,  and  the  reverse. 

J.  Is  the  moon  then  inhabited  as  well  as  the  earth  ? 

T.  Thdugh  we  cannot  demonstrate  this  fact,  yet 


128 


ASTRONOMY. 


there  are  many  reasons  to  induce  us  to  believe  it ;  for 
the  moon  is  a  secondary  planet  of  considerable  size  ; 
—its  surface  is  diversified  like  that  of  the  earth  with 
mountains  and  valleys  : — the  former  have  been  mea- 
sured by  Dr.  Herschel,  and  some  of  them  found  to 
be  about  a  mile  in  height.  The  situation  of  the  moon, 
with  respect  to  the  sun,  is  much  like  that  of  the  earth, 
and  by  a  rotation  on  her  axis,  and  a  small  inclination 
of  that  axis  to  the  plane  of  her  orbit,  she  enjoys, 
though  not  a  considerable,  yet  an  agreeable  va- 
riety of  day  and  night  and  of  seasons.  To  the  moon, 
our  globe  appears  a  capital  satellite,  undergoing  the 
same  changes  of  illumination  as  the  moon  does  to  the 
earth.  The  sun  and  stars  rise  and  set  there  as  they 
do  here,  and  heavy  bodies  will  fall  on  the  moon  as 
they  do  on  the  earth.  Hence  we  are  led  to  conclude 
that,  like  the  earth,  the  moon  also  is  inhabited.  Dr. 
Herschel  discovered  some  years  ago  three  volcanoes, 
ail  burning,  in  the  moon ;  but  no  large  seas  or  tracks 
of  water  have  been  observed  there,  nor  is  the  exist- 
ence of  a  lunar  atmosphere  certain.  Therefore,  her 
inhabitants  must  materially  differ  from  those  who  live 
upon  the  earth. 


CONVERSATION  XV. 

OF  ECLIPSES. 

C.  Will  you,  sir,  explain  to  us  the  nature  and 
causes  of  eclipses  ? 

T.  I  will,  with  great  pleasure.  You  must  observe, 
then,  that  eclipses  depend  upon  this  simple  principle, 
that  all  opaque  or  dark  bodies,  when  exposed  to  any 
light,  and  therefore  to  the  light  of  the  sun,  cast  a 
shadow  behind  them  in  an  opposite  direction. 

J.  The  earth  being  a  body  of  this  kind  must  cast  a 
very  large  shadow  on  its  side  which  is  opposite  to  the 
sun. 


OF  ECLIPSES.  129 
T.  It  does  :  and  an  eclipse  of  the 
moon  happens  when  the  earth  t  passes 
between  the  sun  s  and  the  moon  m, 
and  it  is  occasioned  by  the  earth's 
shadow  being  cast  on  the  moon. 

C.  When  does  this  happen  ? 

T,  It  is  only  when  the  moon  is  full, 
or  in  opposition,  that  it  comes  within 
the  shadow  of  the  earth. 

J.  Eclipses  of  the  moon,  however, 
do  not  happen  every  time  it  is  full  3 
what  is  the  reason  of  this  ? 

T,  Because  the  orbit  of  the  moon 
does  not  coincide  with  the  plane  of  the 
earth's  orbit,  but  one  half  of  it  is  ele- 
vated about  five  degrees  and  a  third  above  it,  and  the 
other  half  is  as  much  below  it :  and  therefore,  unless 
the  full  moon  happen  in  or  near  one  of  the  nodes,  that 
is,  in  or  near  the  points  in  which  the  two  orbits  intersect 
each  other,  she  will  pass  above  or  below  the  shadov/ 
of  the  earth,  in  which  case  there  can  be  no  eclipse. 

C.  What  is  the  greatest  distance  from  the  node,  at 
which  an  eclipse  of  the  moon  can  happen  ? 

T.  There  can  be  n-o  eclipse  if  the  moon,  at  the  time 
when  she  is  full,  be  more  than  12  degrees  from  the 
node  ;  when  she  is  within  that  distance,  there  will  be 
a  partial,  or  total  eclipse,  according  as  a  part,  or  the 
whole  disc  or  face  of  the  moon  falls  within  the  earth's 
shadow.  If  the  eclipse  happen  exactly  when  the 
moon  is  full  in  the  node,  it  is  called  a  central 
eclipse. 

J,  I  suppose  the  duration  of  the  eclipse  lasts  all  the 
tim.e  that  the  moon  is  passing  through  the  shadow. 

T.  It  does  :  and  you  observe  that  the  shadow  is 
considerably  wider  than  the  moon!s  diameter,  and 
therefore  an  eclipse  of  the  moon  lasts  sometimes  three 

I  or  four  hours.    The  shadow  also  you  perceive  is  of  a 

II  conical  shape,  and  consequently,  as  the  moon's  orbit 
is  an  ellipse,  and  not  a  circle,  the  moon  will,  at  differ- 

G  2 


130  ASTRONOMY. 

ent  times,  be  eclipsed  when  she  is  at  different  dis- 
tances from  the  earth. 

C.  And  according  as  the  moon  is  nearer  to,  or 
farther  from  the  earth,  the  eclipse  will  be  of  a  greater 
or  less  duration  ;  for  the  shadow  being  conical  be- 
comes less  and  less,  as  the  distance  from  the  body 
by  which  it  is  cast  is  greater. 

r.  It  is  by  knowing  exactly  at  what  distance  the 
moon  is  from  the  earth,  and  of  course  the  width  of 
the  earth's  shadow  at  that  distance,  that  all  eclipses 
are  calculated,  with  the  greatest  accuracy,  for  many 
years  before  they  happen.  Now,  it  is  found  that  in 
all  eclipses  the  shadow  of  the  earth  is  conical,  which 
is  a  demonstration,  that  the  body  by  which  it  is  pro- 
jected is  of  a  spherical  form,  for  no  other  sort  of  figure 
would,  in  all  positions,  cast  a  conical  shadow.  This 
is  mentioned  as  another  proof,  that  the  earth  is  a 
spherical  body, 

J.  It  seems  to  me  to  prove  another  thing,  viz.  that 
the  sun  must  be  a  larger  body  than  the  earm. 

T.  Your  conclusion  is  just,  for  if  the  two  bodies 
were  equal  to  one  another  the  shadow  would  be 


Fig.  13.  Fig.  14. 


cylindrical ;  and  if  the  earth  were  the  larger  body, 
its  shadow  would  be  the  figure  of  a  cone,  which  had 
lost  its  vertex,  and  the  farther  it  were  extended  the 
larger  would  it  become.  In  either  case  the  shadow 
would  run  out  to  an  infinite  space,  and  accordingly 
must  sometimes  involve  in  it  the  other  planets,  and 
eclipse  them,  which  is  contrary  to  fact.  Therefore, 
since  the  earth  is  neither  larger  than,  nor  equal  to, 
the  sun,  it  must  be  the  lesser  body. — We  will  now 
proceed  to  the  eclipses  of  the  sun, 
C.  How  are  these  occasioned  ? 


OF  ECLIPSES,  13X 
T.  An  eclipse  of  the 
sun  happens  when  the 
moon  M,  passing  between 
the  sun  s  and  the  earth 
T,  intercepts  the  sun's 
light. 

J.  The  sun  then  can 
be  eclipsed  only  at  the  new  moon  ? 

T,  Certainly ;  for  it  is  only  when  the  moon  is  in 
conjunction,  that  it  can  pass  directly  between  the  sun 
and  earth. 

C.  Is  it  only  when  the  moon  at  her  conjunction  is 
near  one  of  its  nodes,  that  there  can  be  an  eclipse  of 
the  sun  ? 

T.  An  eclipse  of  the  sun  depends  upon  this  circum- 
stance :  for  unless  the  moon  is  in,  or  near,  one  of  its 
nodes,  she  cannot  appear  in  the  same  plane  with  the 
sun,  or  seem  to  pass  over  his  disc.  In  every  other 
part  of  the  orbit  she  will  appear  above  or  below  the 
sun.  If  the  moon  be  in  one  of  the  nodes  she  will,  in 
most  cases,  cover  the  whole  disc  of  the  sun,  and  pro- 
duce a  total  eclipse;  if  she  be  any  where  within 
about  16  degrees  of  a  node,  a  partial  eclipse  will  be 
produced. 

The  sun's  diameter  is  supposed  to  be  divided  into 
12  equal  parts  called  digits,  and  in  every  partial 
eclipse,  as  many  of  these  parts  of  the  sun's  diameter 
as  the  moon  covers,  so  many  digits  are  said  to  be 
eclipsed. 

J.  I  have  heard  of  annular  eclipses;  what  are 
they,  sir? 

T.  When  a  ring  of  light  appears  round  the  edge  of 
the  moon  during  an  eclipse  of  the  sun,  it  is  said  to  be 
annular,  from  the  Latin  word  annulus,  a  ring ;  these 
kind  of  eclipses  are  occasioned  by  the  moon  being  at 
her  greatest  distance  from  the  earth  at  the  time  of  an 
eclipse,  because,  in  that  situation,  the  vertex  or  tip  of 
the  cone  of  the  moon's  shadow  does  not  reach  the 
surface  of  the  earth. 

I    C.  How  long  can  an  eclipse  of  the  sun  last  ? 


132  ASTRONOMY. 

T.  A  total  eclipse  of  tlie  sun  is  a  very  curious  and 
uncommon  spectacle ;  and  total  darkness  cannot  last 
more  than  three  or  four  minutes.  Of  one  that  was 
observed  in  Portugal  180  years  ago,  it  is  said  that  the 
darkness  was  greater  than  that  of  the  night ; — that 
stars  of  the  first  magnitude  made  their  appearance  ; — 
and  that  the  birds  were  so  terrified  that  they  fell  to  the 
ground. 

J.  Was  this  visible  only  at  Portugal  ? 

T.  It  must  have  been  seen  at  other  places,  though 
we  have  no  account  of  it.  The  moon  being  a  body 
much  smaller  than  the  earth,  and  having  also  a  coni- 
cal shadow,  can  with  that  shadow  only  cover  a  small 
part  of  the  earth  ;  whereas  an  eclipse  of  the  moon 
may  be  seen  by  all  those  that  are  on  that  hemisphere 
which  is  turned  towards  it.  (Figs.  15.  and  12.)  You 
will  also  observe,  that  an  eclipse  of  the  sun  may 
be  total  to  the  inhabitants  near  the  middle  of  the 
earth's  disc,  and  annular  to  those  in  places  near  the 
edges  of  the  disc  ;  for  in  the  former  case  the  moon's 
shadow  will  reach  the  earth,  and  in  the  latter,  on 
account  of  the  earth's  sphericity,  it  will  not. 

C.  Have  not  eclipses  been  esteemed  as  omens  pre- 
saging some  direful  calamity  ? 

r.  Till  the  causes  of  these  appearances  were  dis- 
covered, they  were  generally  beheld  with  terror  by 
the  inhabitants  of  the  world. 

CONVERSATION  XVI. 

OF  THE  TIDES. 

T.  We  will  proceed  to  the  consideration  of  the 
Tides,  or  the  flowing  and  ebbing  of  the  ocean. 

J.  Is  this  subject  connected  with  astronomy  ? 

r.  It  is,  inasmuch  as  the  tides  are  occasioned  by 
the  attraction  of  the  sun  and  moon  upon  the  waters, 
but  more  particularly  by  that  of  the  latter.  You  will 
readily  conceive  that  the  tides  are  dependent  upon 
some  known  and  determinate  laws,  because,  if  you  turn 
to  the  Ephemeris,  or  indeed  to  almost  any  almanac, 


OF  THE  TIDES. 


133 


you  will  see  that  the  exact  tune  of  high  water  at 
London-bridge  on  the  morning  and  afternoon  of  every 
day  in  the  year  is  set  down. 

C.  I  have  frequently  wondered  how  this  could  be 
known  with  such  a  degree  of  accuracy  :  indeed  there 
is  not  a  waterman  that  plies  at  the  stairs  but  can 
readily  tell  when  it  will  be  high  water. 

r.  The  generality  of  the  watermen  are  probably 
as  ignorant  as  yourself  of  the  cause  by  which  the 
waters  flow  and  ebb ;  but  by  experience  they  know  that 
the  time  of  high  water  diflfers  on  each  day  about  three 
quarters  of  an  hour,  or  a  little  more  or  less,  and  there- 
fore, if  it  be  high  water  to-day  at  six  o'clock,  they 
will,  at  a  guess,  tell  you,  that  to-morrow  the  tide  will 
not  be  up  till  a  quarter  before  seven. 

/.  Will  you  explain  the  causes  ? 

r.  I  will  endeavour  to  do  this  in  an  easy  and  con- 
cise manner,  without  fatiguing  your  memory  with  a 
great  variety  of  particulars. 

You  must  bear  in  your  mind,  then,  that  the  tides 
are  occasioned  by  the  attraction  of  the  sun  and  moon 
upon  the  waters  of  the  earth  :  perhaps  a  figure  may  be 


Fig.  16. 

of  some  assistance  to  you.  Let  a^ln  be  supposed  the 
earth,  c  its  centre  ;  let  the  dotted  circle  represent  a 
mass  of  water  covering  the  earth  :  let  m  be  the  moon 
in  its  orbit ;  and  s  the  sun. 

Since  the  force  of  gravity  or  attraction  diminishes 
as  the  squares  of  the  distances  increase,*  the  waters 


*  See  Mechanics,  Conversation  VIL 


134 


ASTRONOMY. 


on  the  side  a  are  more  attracted  by  the  moon  m,  than 
the  central  parts  at  c ;  and  the  central  parts  are 
more  attracted  than  the  waters  at  / ;  consequently 
the  waters  at  d  will  recede  from  the  centre ;  there- 
fore, while  the  moon  is  in  the  situation  m,  the  waters 
will  rise  towards  h  and  d  on  the  opposite  sides  of  the 
earth. 

C.  You  mean  that  the  waters  will  rise  at  d  by  the 
immediate  attraction  of  the  moon  n?,  and  will  rise  at 
h,  by  the  centre  c  receding  and  leaving  them  more 
elevated  there, 

r.  That  is  the  explanation.  It  is  evident  that  the 
quantity  of  water  being  the  same,  a  rise  cannot  take 
place  at  b  and  d,  without  the  parts  at  e  and  /  being 
at  the  same  time  depressed. 

J.  In  this  situation  the  water  may  be  considered  as 
partaking  of  an  oval  form. 

T.  If  the  earth  and  moon  were  without  motion, 
and  the  earth  covered  all  over  with  water,  the  attrac- 
tion of  the  moon  would  raise  it  up  in  a  heap  in  that 
part  of  the  ocean  to  which  the  moon  is  vertical, 
and  there  it  would  always  continue  ;  but,  by  the  ro- 
tation of  the  earth  on  its  axis,  each  part  of  its  surface 
to  which  the  moon  is  vertical  is  presented  twice  a  day 
to  the  action  of  the  moon,  and  thus  are  produced  two 
floods  and  two  ebbs. 

C.  How  twice  a  day  1 

T.  In  the  position  of  the  earth  and  moon  as  it 
is  in  our  figure,  the  waters  are  raised  at  d  by  the 
direct  attraction  of  the  moon,  and  a  tide  is  accordingly 
produced  :  but  when,  by  the  earth's  rotation,  a  comes, 
12  hours  afterwards,  into  the  position  /,  another  tide 
is  occasioned  by  the  receding  of  the  waters  there  from 
the  centre. 

J.  You  have  told  us  that  the  tides  are  produced 
in  those  parts  of  the  earth  to  which  the  moon  is  verti- 
cal ;  but  this  effect  is  not  confined  to  those  parts  ? 

T.  It  is  not ;  but  there  the  attraction  of  the  moon 
has  the  greatest  eflfect ;  in  all  other  parts  the  force  is 
weaker,  because  it  acts  in  a  more  oblique  direction. 


OF  THE  TIDES. 


135 


C.  Are  there  two  tides  in  every  24  hours  ? 

T,  If  the  moon  were  stationary  this  would  be  the 
case  ;  but  because  that  body  is  also  proceeding  every 
day  about  13  degrees  from  west  to  east  in  her  orbit, 
the  earth  must  make  more  than  one  revolution  on  its 
axis  before  the  same  meridian  is  in  conjunction  with 
the  moon,  and  hence  two  tides  take  place  in  about  24 
hours  and  50  minutes. 

J.  But  I  remember  when  we  were  at  the  sea,  that 
the  tides  rose  higher  at  some  seasons  than  at  others  : 
how  do  you  account  for  this  1 

T.  The  moon  goes  round  the  earth  in  an  elliptic 
orbit,  and  therefore  she  approaches  nearer  to  the 
earth  in  some  parts  of  her  orbit  than  in  others.  When 
she  is  nearest,  the  attraction  is  the  strongest,  and  cori- 
sequently  it  raises  the  tides  most :  and  when  she  is 
farthest  from  the  earth,  her  attraction  is  the  least,  and 
the  tides  the  lowest. 

/.  Do  they  rise  to  different  heights  in  different 
places  1 

T.  They  do  :  in  the  Black  Sea  and  the  Medi- 
terranean the  tides  are  scarcely  perceptible.  At  the 
mouth  of  the  Indus  the  water  rises  and  falls  full  30 
feet.  The  tides  are  remarkably  high  on  the  coasts  of 
Malay,  in  the  Straits  of  Punda,  in  the  Ked  Sea,  along 
the  coasts  of  China,  Japan,  &c.  In  general,  the 
tides  rise  highest  and  strongest  in  those  places  that  are 
narrowest. 

C.  You  said  the  sun's  attraction  occasioned  tides 
as  well  as  that  of  the  moon. 

r.  It  does  ;  but,  owing  to  the  immense  distance  of 
the  sun  from  the  earth,  it  produces  but  a  small  effect 
in  comparison  of  the  moon's  attraction.  Sir  Isaac 
Newton  computed  that  the  force  of  the  moon  raised 
the  waters  in  the  great  ocean  10  feet,  whereas  that  of 
the  sun  raised  it  only  2  feet.  When  both  the  attrac- 
tions of  the  sun  and  moon  act  in  the  same  direction, 
that  is  at  new  and  full  moon,  the  combined  forces  of 
both  raise  the  tide  12  feet.  But  when  the  moon  is  in 
her  quarters,  the  attraction  of  one  of  these  bodies 


136 


ASTRONOMY. 


raises  the  water,  while  that  of  the  other  depresses  it ; 
and  therefore  the  smaller  force  of  the  sun  must  be 
subtracted  from  that  of  the  moon,  consequently  the 
tides  will  be  no  more  than  8  feet.  The  highest  tides 
are  called  spring  tides,  and  the  lowest  are  denominated 
neap  tides. 

J.  I  understand  that,  in  the  former  case,  the  height 
to  which  the  tides  are  raised  must  be  calculated  by 
adding  together  the  attractions  of  the  sun  and  moon , 
and  in  the  latter,  it  must  be  estimated  by  the  differ- 
ence of  these  attractions. 

T,  You  are  right.  When  the  sun  and  moon  are 
both  vertical  to  the  equator  of  the  earth,  and  the 
moon  at  her  least  distance  from  the  earth,  then  the 
tides  are  highest. 

C.  Do  the  highest  tides  happen  at  the  equinoxes  1 

T,  Strictly  speaking,  these  tides  do  not  happen  till 
some  little  time  after,  because  in  this,  as  in  other 
cases,  the  actions  do  not  produce  the  greatest  effect 
when  they  are  at  the  strongest,  but  some  time  after- 
wards ;  thus  the  hottest  part  of  the  day  is  not  when 
the  sun  is  on  the  meridian,  but  between  two  and  four 
o'clock  in  the  afternoon. — Another  circumstance  must 
be  taken  into  consideration  :  the  sun  being  nearer  to 
the  earth  in  winter  than  in  summer,  it  is  of  course 
nearer  to  it  in  February  and  October,  than  in  IMarch 
and  September  ;  and  therefore,  all  these  things  being 
*put  together,  it  will  be  found  that  the  greatest  tides 
happen  a  little  before  the  vernal,  and  some  time  after 
the  autumnal,  Equinoxes. 

As  the  tides  are  more  affected  by  the  attraction  of 
the  moon  than  by  that  of  the  sun,  the  magnitude  of 
the  tides  varies  with  the  distance  of  the  moon  from 
the  earth  ;  the  attraction  is  also  greatest  when  she  is 
in  her  perigee,  or  nearest  the  earth ;  and  it  is  least 
when  she  is  in  her  apogee,  or  the  point  farthest  from 
the  earth. 


OF  THE  HARVEST  MOON. 


137 


CONVERSATION  XVII. 

OF  THE  HARVEST  MOON. 

T.  From  what  we  said  yesterday,  you  will  easily 
understand  the  reason  why  the  moon  rises  about  three 
quarters  of  an  hour  later  every  day  than  on  the  one 
preceding. 

C.  It  is  owing  to  the  daily  progress  which  the  moon 
is  making  in  her  orbit,  on  which  account  any  meridian 
on  the  earth  must  make  more  than  one  complete  rota- 
tion on  its  axis,  before  it  comes  again  into  the  same 
situation  with  respect  to  the  moon  that  it  had  before. 
And  you  told  us  that  this  occasioned  a  difference  of 
about  50  minutes. 

'  T.  At  the  equator  this  is  generally  the  difFerence 
of  time  between  the  rising  of  the  moon  on  one  day 
and  the  preceding.  But  in  places  of  considerable 
latitude,  as  that  in  which  we  live,  there  is  a  remarkable 

'  dilference  about  the  time  of  harvest,  when  at  the  sea- 
son of  full  moon  she  rises  for  several  nights  together 
only  about  20  minutes  later  on  the  one  day  than  on 

i  that  immediately  preceding.    By  thus  succeeding  the 

1}  sun  before  the  twilight  is  ended,  the  moon  prolongs 

{  the  light,  to  the  great  benefit  of  those  who  are  engaged 
in  gathering  in  the  fruits  of  the  earth  ;  and  heifte  the 
full  moon  at  this  season  is  called  the  harvest  moon. 

!  It  is  believed  that  this  was  observed  by  persons  en- 
gaged in  agriculture,  at  a  much  earlier  period  than  it 

;  was  noticed  by  astronomers  ;  the  former  ascribed  it 
to  the  goodness  of  the  Deity,  not  doubting  but  that 

I  he  had  so  ordered  it  on  purpose  for  their  advantage. 
J.  But  the  people  at  the  equator  do  not  enjoy  this 
benefit. 

i  T.  Nor  is  it  necessary  that  they  should,  for  in 
:  those  parts  of  the  earth  the  seasons  vary  but  little, 
and  the  weather  changes  but  seldom,  and  at  stated 
i  times ;  to  them,  then,  moon-light  is  not  wanting  for 
i  gathering  the  fruits  of  the  earth. 


138 


ASTRONOMY. 


C.  Can  you  explain  how  it  happens,  that  the  moon 
at  this  season  of  the  year  rises  one  day  after  another 
with  so  small  a  difference  of  time  ? 

r.  With  the  assistance  of  a  globe  I  could  at  once 
clear  the  matter  up.  But  I  will  endeavour  to  give 
you  a  general  idea  of  the  subject  without  that  instru- 
ment. That  the  moon  loses  more  time  in  her  risings 
when  she  is  in  one  part  of  her  orbit,  and  less  in 
another,  is  occasioned  by  the  moon's  orbit  lying  some 
times  more  oblique  to  the  horizon  than  at  others. 

J.  But  the  moon's  path  is  not  marked  on  the 
globe. 

T.  It  is  not  ]  you  may,  however,  consider  it,  with- 
out much  error,  as  coinciding  with  the  ecliptic.  And 
in  the  latitude  of  London,  as  much  of  the  ecliptic 
rises  about  Pisces  and  Aries  in  two  hours  as  the  moon 
goes  through  in  six  days  ;  therefore,  while  the  moon  is 
in  these  signs  she  differs  but  two  hours  in  rising  for  six 
days  together  ;  that  is,  one  day  with  another,  about 
20  minutes  later  every  day  than  on  the  preceding. 

C.  Is  the  moon  in  those  signs  at  the  time  of  harvest  ? 

r.  In  August  and  September  you  know  that  the  sun 
appears  in  Virgo  and  Libra,  and,  of  course,  when  the 
moon  is  full,  she  must  be  in  the  opposite  signs,  viz. 
Pisces  and  Aries, 

C.  Will  you  explain,  sir,  how  it  is  that  the  people 
at  the  equator  have  no  harvest  moon  ? 

T.  At  the  equator,  the  north  and  south  poles  lie  in 
the  horizon,  and  therefore  the  ecliptic  makes  the  same 
angle  southward  with  the  horizon  when  Aries  rises,  as 
it  does  northward  when  Libra  rises ;  but  as  the  har- 
vest moon  depends  upon  the  different  angles  at  which 
different  parts  of  the  ecliptic  rises,  it  is  evident  tliere 
can  be  no  harvest  moon  at  the  equator. 

The  farther  any  place  is  from  the  equator,  if  it  be 
not  beyond  the  polar  circles,  the  angle  which  the 
ecliptic  makes  with  the  horizon,  when  Pisces  and 
Aries  rise,  gradually  diminishes,  and  therefore  when 
the  moon  is  in  these  signs  she  rises  with  a  nearly 
proportionable  diliercnce  later  every  day  than  on  the 


OF  THE  HARVEST  MOON.  139 
former,  and  this  is  more  remarkable  about  the  time  of 
full  moon. 

J.  Why  have  you  excepted  the  space  on  the  globe 
beyond  the  polar  circles  ? 

At  the  polar  circles,  when  the  sun  touches  the 
summer  tropic  he  continues  24  hours  above  the  hori^ 
zon,  and  24  hours  below  it  when  he  touches  the 
winter  tropic.  For  the  same  reason  the  full  moon 
neither  rises  in  the  summer,  when  she  is  not  wanted, 
nor  sets  in  the  winter,  when  her  presence  is  so  neces- 
sary. These  are  the  only  two  full  moons  which  hap- 
pen about  the  tropics ;  for  all  the  others  rise  and  set. 
In  summer  the  full  moons  are  low,  and  their  stay 
above  the  horizon  short :  in  winter  they  are  high,  and 
stay  long  above  the  horizon.  A  wonderful  display 
this  of  the  divine  wisdom  and  goodness,  in  apportion- 
ing the  quantity  of  light  suitable  to  the  various  neces- 
sities of  the  inhabitants  of  the  earth,  according  to  their 
different  situations. 

C.  At  the  poles,  the  matter  is,  I  suppose,  still  dil- 
ferent. 

r.  There  one  half  of  the  ecliptic  never  sets  and  the 
other  half  never  rises  ;  consequently  the  sun  con- 
tinues one  half  year  above  the  horizon,  and  the  other 
half  below  it.  The  full  moon  being  always  opposite 
to  the  sun  can  never  be  seen  to  the  inhabhants  of  the 
poles,  while  the  sun  is  above  the  horizon.  But  all 
the  time  that  the  sun  is  below  the  horizon,  the  ful 
moons  never  set.  Consequently  to  them  the  full 
moon  is  never  visible  in  their  summer  ;  and  in  their 
winter  they  have  her  always  before  and  after  the  full, 
shining  for  14  of  our  days  and  nights  without  inter- 
mission. And  when  the  sun  is  depressed  the  lowest 
under  the  horizon,  then  the  moon  ascends  with  her 
highest  altitude. 

J.  This  indeed  exhibits  in  a  high  degree  the  atten- 
tion of  Providence  to  all  his  creatures.  But  if  I 
understand  you,  the  inhabitants  of  the  poles  have 
ill  their  winter  a  fortnight's  light  and  darkness  by 
turns  ? 


140 


ASTRONOMY. 


T.  This  would  be  the  case  for  the  whole  six 
months  that  the  sun  is  below  the  horizon,  if  there 
were  no  refraction,*  and  no  substitute  for  the  light  of 
the  moon.  But  by  the  atmosphere's  refracting  the 
sun's  rays,  he  becomes  visible  a  fortnight  sooner,  and 
continues  a  fortnight  longer  in  sight,  than  he  would 
otherwise  do  were  there  no  such  property  belonging 
to  the  atmosphere.  And  in  those  parts  of  the  winter, 
when  it  would  be  absolutely  dark  in  the  absence  of 
the  moon,  the  brilliancy  of  the  Aurora  Boreatis  is 
probably  so  great  as  to  afford  a  very  comfortable  de- 
gree of  light.  Mr.  Hearne,  in  his  travels  near  the 
polar  circle,  has  this  remark  in  his  journal :  "  De- 
cember 24.  The  days  were  so  short,  that  the  sun 
only  took  a  circuit  of  a  few  points  of  the  compass 
above  the  horizon,  and  did  not  at  its  greatest  altitude 
rise  half  way  up  the  trees.  The  brilliancy  of  the 
Aurora  Borealis,  however,  and  of  the  stars,  even 
without  the  assistance  of  the  moon,  made  amends  for 
this  deficiency,  for  it  was  frequently  so  light  all  night, 
that  I  could  see  to  read  a  small  print." 


CONVERSATION  XVIII. 

OF  MERCURY. 

T.  Having  fully  described  the  earth  and  the  moon, 
the  former  a  primary  planet,  and  the  latter  its  at- 
tendant satellite,  or  secondary  planet,  we  shall  next 
consider  the  other  planets  in  their  order,  with  which, 
however,  we  are  less  interested. 

Mercury,  yon  recollect,  is  the  planet  nearest  the 
sun  ;  and  Venus  is  the  second  in  order.  These  are 
called  inferior  planets. 

C.  Why  are  they  thus  denominated? 

T.  Because  they  both  revolve  in  orbits  which  are 
included  ivithin  that  of  the  earth  ;  thus  (Fig.  2.  p.  93.) 

*  The  subject  of  refraction  will  be  very  particularly 
explained  when  we  come  to  Optics. 


OF  MERCURY. 


141 


Mercury  makes  bis  annual  journey  round  the  sun  in 
the  orbit  a ;  Venus  in  l>,  and  the  earth,  farther  from 
that  luminary  than  either  of  them,  makes  its  circuit 
in  t. 

J.  How  is  this  known  1 

T.  By  observation  :  for  by  attentively  watching 
the  progress  of  these  bodies,  it  is  found  that  they  are 
continually  changing  their  places  among  the  fixed 
stars,  and  that  they  are  never  seen  in  opposition  to  the 
sun,  that  is,  they  are  never  seen  in  the  western  side  of 
the  heavens  in  the  morning  when  he  appears  in  the 
east ;  nor  in  the  eastern  part  of  the  heavens  in  the 
evening  when  the  sun  appears  in  the  west. 

C.  Then  they  may  be  considered  as  attendants  upon 
the  sun  7 

T.  They  may  :  Mercury  is  never  seen  from  the 
earth  at  a  greater  distance  from  the  sun  than  about  28 
degrees,  or  about  as  far  as  the  moon  appears  to  be 
from'  the  sun  on  the  second  day  after  its  change  ; 
hence  it  is  that  we  so  seldom  see  him  ;  and  when  we  do, 
it  is  for  so  short  a  time,  and  always  in  twilight,  that 
sufficient  observations  have  not  been  made  to  ascertain 
whether  he  has  a  diurnal  motion  on  his  axis. 

J.  Would  you  then  conclude  that  he  has  such  a 
motion  1 

T.  i  think  we  ought ;  because  it  is  known  to  exist 
in  all  those  planets  upon  which  observations  of  suf- 
ficient extent  have  been  made,  and  therefore  we  may 
surely  infer,  without  much  chance  of  error,  that  it  be- 
longs also  to  Mercury,  and  the  Hevschel  planet ;  the 
former  from  its  vicinity  to  the  sun,  and  the  latter 
from  its  great  distance  from  that  body,  having  at 
present  eluded  the  investigation  of  the  most  indefati- 
gable astronomers. 

C.  At  what  distance  is  Mercury  from,  the  sun  ? 

T.  He  revolves  round  that  body  at  about  37  mil- 
lions of  miles  distance,  in  88  days  nearly  ;  and  there- 
fore you  can  now  tell  me  how  many  miles  he  travels 
in  an  hour. 

J.  I  can  ;  for  supposing  his  orbit  circular,  I  must 


142  ASTRONOMY. 

multiply  the  37  millions  by  6,*  which  will  give  222 
millions  of  miles  for  the  length  of  his  orbit ;  this  I 
shall  divide  by  88,  the  number  of  days  he  takes  in  per- 
forming his  journey,  and  the  quotient  resulting  from 
this  must  be  divided  by  24,  for  the  number  of  hours  in 
a  day  ;  and  by  these  operations  I  find,  that  Mercury 
travels  at  the  rate  of  more  than  105,000  miles  in  an 
hour. 

C.  How  large  is  Mercury  ? 

T.  He  is  the  sm.allest  of  all  the  planets.  His  dia- 
meter is  something  more  than  3200  miles  in  length. 

J,  His  situation  being  so  much  nearer  to  the  sun 
than  ours,  he  must  enjoy  a  considerably  greater  share 
of  its  heat  and  light. 

T.  So  much  so,  as  would  indeed  infallibly  burn 
every  thing  belonging  to  the  earth  to  atoms,  were  she 
similarly  situated.  The  heat  of  the  sun  at  Mercury, 
must  be  7  times  greater  than  our  summer  heat. 

C.  And  do  you  imagine  that,  thus  circumstanced, 
this  planet  can  be  inhabited  ? 

T.  N ot  by  such  beings  as  we  are  ;  you  and  I  could 
not  long  exist  at  the  bottom  of  the  sea ;  yet  the  sea  is 
the  habitation  of  millions  of  living  creatures;  why 
then  may  there  not  be  inhabitants  in  Mercury,  fitted 
for  the  enjoyment  of  the  situation  which  that  planet  is 
calculated  to  afford  ?  If  there  be  not,  we  must  be  at 
a  loss  to  know  why  such  a  body  was  formed  ;  certamly 
it  could  not  be  intended  for  our  benefit,  for  it  is  rarely 
even  seen  by  us. 

CONVERSATION  XIX. 

OF  VENUS. 

T.  We  now  proceed  to  Venus,  the  second  planet 
in  the  order  of  the  solar  system,  but  by  far  the  most 
beautiful  of  them  all. 

/.  How  far  is  Venus  from  the  sun  ? 


*  Sec  Conver.  VU.  Ae>troiiomy. 


OF  VENUS.  143 
T,  That  planet  is  68  millions  of  miles  from  the  sun, 
and  she  finishes  her  journey  in  224|  days,  conse- 
quently she  must  travel  at  the  rate  of  75,000  miles  m 
an  hour. 

C.  Venus  is  larger  than  Mercury,  I  dare  say. 

T.  Yes,  she  is  nearly  as  large  as  the  earth,  which 
she  resembles  also  in  other  respects,  her  diameter  bemg 
about  7700  miles  in  length,  and  she  has  a  rotation 
about  her  axis  in  23  hours  and  20  minutes.  The 
quantity  of  light  and  heat  which  she  enjoys  from  the 
sun,  must  be  double  that  which  is  experienced  by  the 
inhabitants  of  this  globe. 

J.  Is  there  also  a  difference  m  her  seasons,  as  there 
is  here  1 

T.  Yes,  in  a  much  more  considerable  degree. 
The  axis  of  Venus  inclines  about  75  degrees,  but  that 
of  the  earth  inclines  only  23 1  degrees,  and  as  the 
variety  of  the  seasons  in  every  planet  depends  on  the 
degree  of  the  inclination  of  its  axis,  it  is  evident  that 
the  seasons  must  vary  more  with  Venus  than  with  us. 

C.  Venus  appears  to  us  larger  sometimes  than  at 
others. 

r.  She  does;  and  this,  with  other  particulars,  I 
will  explain  by  means  of  a  figure.    Suppose  s  to  be 


Fig.  17. 

the  sun,  t  the  earth  in  her  orbit,  and  a,  h,  c,  d,  /, 


14i 


ASTRONOMY. 


V enus  in  her's  :  now  it,  is  evident,  that  when  Venus  is 
at  a  between  the  sun  and  earth,  she  wouhi,  if  visible, 
appear  much  larger  than  when  she  is  at  d,  in  oppo- 
sition. 

J.  That  is  because  she  is  so  much  nearer  in  the 
former  case  than  in  the  latter,  being  in  the  situation  a 
but  27  millions  of  miles  from  the  earth  t,  but  at  d  she 
is  163  millions  of  miles  off. 

T,  Now  as  Venus  passes  from  a,  through  h,  c,  to  d, 
she  may  be  observed,  by  means  of  a  good  telescope, 
to  have  all  the  same  phases  as  the  moon  has  in  pass- 
ing from  new  to  full ;  therefore,  when  she  is  at  d  she 
is  full,  and  is  seen  among  the  fixed  stars  ;  during  her 
journey  from  d  to  e,  she  proceeds  with  a  direct  motion 
in  her  orbit,  and  at  e  she  will  appear  to  an  inhabitant 
of  the  earth,  for  a  few  days,  to  be  stationary,  not  seem- 
ing to  change  her  place  among  the  fixed  stars,  for  she 
is  coming  toward  the  earth  in  a  direct  line  :  but  in 
passing  from  e  to/,  though  still  with  a  direct  motion, 
yet,  to  a  spectator  at  t,  her  course  will  seem  to  be 
back  again,  or  retrograde,  for  she  will  seem  to  have 
gone  back  from  x  to  y ;  her  path  will  appear  retro- 
grade till  she  gets  to  c,  when  she  will  again  appear 
stationary,  and  afterwards,  from  c  to  d,  and  from  d 
to  e,  it  will  be  direct  among  the  fixed  stars. 

C.  When  is  Venus  an  evening  and  when  a  morn- 
ing star  1 

T,  She  is  an  evening  star  all  the  while  she  appears 
east  of  the  sun,  and  a  morning  star  while  she  is  seta 
west  of  him.  When  she  is  at  a  she  will  be  mvi^-ib'e, 
her  dark  side  being  towards  us,  unless  she  be  exactly 
in  the  node,  in  which  case  she  will  pass  over  the  sun's 
face  like  a  little  black  spot. 

J.  Is  that  called  the  transit  of  Venus  1 
r.  It  is  ;  and  it  happens  twice  only  in  about  120 
years.  By  this  phenomenon  astronomers  have  been 
enabled  to  ascertain  with  great  accuracy  the  distance 
of  the  earth  from  the  sun  j  and,  having  obtained  this, 
the  distances  of  the  other  planets  are  easily  found. 
By  the  two  transits  which  happened  in  1761,  and 


OF  VENUS.' 


145 


1769,  it  was  clearly  demonstrated,  that  the  mean  dis- 
tance of  the  earth  from  the  sun  was  between  95  and 
96  millions  of  miles.  The  next  transit  of  Venus  will 
be  in  1874. 

C.  How  do  you  find  the  distances  of  the  other 
planets  from  the  sun,  by  knowing  that  of  the  earth  1* 
T.  1  will  endeavour  to  make  this  plain  to  you. 
Kepler,  a  great  astronomer,  discovered  that  all  the 
planets  are  subject  to  one  general  law,  which  is,  that 
the  squares  of  their  periodical  times  are  proportional 
to  the  cubes  of  their  distances  from  the  sun, 

J.  What  do  you  mean  by  the  periodical  times  ? 
T.  I  mean  the  times  which  the  planets  take  m  re- 
volving round  the  sun  ;  thus  the  periodical  time  of 
j  the  earth  is  365|  days ;  that  of  Venus  224|  days  ; 
1  that  of  Mercury  88  days. 

'>  C.  How  then  would  you  find  the  distance  of  Mer- 
i  cury  from  the  sun  1 

i  T.  By  the  rule  of  three ;  I  would  say,  as  the 
square  of  365  days  (the  time  which  the  earth  takes  in 
revolving  about  the  sun)  is  to  the  square  of  88  days 
i'  (the  time  in  which  Mercury  revolves  about  the  sun), 
I  so  is  the  cube  of  95  millions  (the  distance  in  miles  of 
j  the  earth  from  the  sun)  to  a  fourth  number. 

J,  And  is  that  fourth  number  the  distance  in  miles 
of  Mercury  from  the  sun  1 

T.  No  :  you  must  extract  the  cube  root  of  that 
number,  and  then  you  will  have  about  37  millions  of 
miles  for  the  answer,  which  is  the  true  distance  at 
which  Mercury  revolves  about  the  sun. 

i  *  The  remainder  of  this  Conversation  may  be  omitted 
t  by  those  youn^  persons  who  are  not  ready  in  arithmetical 
operations.  The  author,  however,  knows  from  experi- 
i;  ence,  that  children  may,  at  a  very  early  age,  be  brought 
i    to  understand  these  higher  parts  of  arithmetic. 


H 


IIG 


ASTRONOMY. 


CONVERSATION  XX. 

OF  MARS. 

T.  Next  to  Venus  is  the  earth  and  her  satellite 
the  moon,  but  of  these  sufficient  notice  has  already 
been  taken,  and  therefore  we  shall  pass  on  to  the 
planet  Mars,  which  is  known  in  the  heavens  by  a 
dusky  red  appearance.  Mars,  together  with  Jupiter, 
Saturn  and  the  Herschel,  are  called  superior  planets, 
because  the  orbit  of  the  earth  is  enclosed  by  their 
orbits. 

C.  At  what  distance  is  Mars  from  the  sun  1 
T.  About  144  millions  of  miles  ;  the  length  of  his 
year  is  equal  to  687  of  our  days,  and  therefore  he 
travels  at  the  rate  of  more  than  53  thousand  miles  in 
an  hour  :  his  diurnal  rotation  on  his-  axis  is  performed 
in  24  hours  and  39  minutes,  which  makes  his  figure 
tliat  of  an  oblate  spheroid. 

J.  How  is  the  diurnal  motion  of  this  planet  dis- 
covered ? 

T.  By  means  of  a  very  large  spot  which  is  seen 
distinctly  on  his  face,  when  he  is  in  that  part  of  his 
orbit  which  is  opposite  to  the  sun  and  earth. 

C.  Is  Mars  as  large  as  the  earth  1 

T.  No  :  his  diameter  is  only  4189  miles  in  length, 
which  is  but  little  more  than  half  the  length  of  the 
earth's  diameter.  And  owing  to  his  distance  from  the 
sun  he  will  not  enjoy  one  half  of  the  light  and  heat 
which  we  enjoy. 

/.  And  yet,  I  believe,  he  has  not  the  benefit  of  a 
moonl 

T.  No  moon  has  ever  been  discovered  belonging 
either  to  Mercury,  Venus,  or  Mars. 

C.  Do  the  superior  planets  exhibit  similar  appear- 
ances of  direct  and  retrograde  motion  to  those  of  the 
inferior  planets  1 

T.  They  do  :  suppose  s  the  sun  ;  a,  h,  d,  /,  ^,  h 


OF  MARS. 


Fig.  18. 

tlie  earth  in  different  parts  of  its  orbit,  and  in  Mars 
in  his  orbit.    When  the  earth  is  at  a,  Mars  will  ap- 
pear among  the  fixed  stars  at  :r.-  when,  by  its  annual 
motion,  the  earth  has  arrived  at  b,  d,  and  /,  respect- 
ively, the  planet  Mars  will  appear  in  the  heavens  at 
|:  y,  Zf  and  w :  when  the  earth  has  advanced  to  g, 
I  Mars  will  appear  stationary  at  o  :  to  the  earth  in  its 
j;  journey  from  g  to  h  the  planet  will  seem  to  go  back- 
I   wards  or  retrograde  in  the  heavens  from  o  to  z,  and 
j  this  retrograde  motion  will  be  apparent  till  the  earth 
I  has  arrived  at  a,  when  the  planet  will  again  appear 
j  stationary. 

!  /.  I  perceive  that  Mars  is  retrograde  when  in  op- 
I  position,  and  the  same  is  I  suppose  applicable  to  the 
f  other  superior  planets,  but  the  retrograde  motion  of 

Mercury  and  Venus  is  when  those  planets  are  in 

conjunction. 

T,  You  are  right  ;  and  you  see  the  reason,  T  dare 
I  say,  why  the  superior  planets  may  be  in  the  west  in 
the  morning  when  the  sun  rises  in  the  east,  and  the 
i  reverse. 

j  C.  For  when  the  earth  is  at  Mars  may  be  at  n, 
I  in  which  case  the  earth  is  between  the  sun  and  the 
I  planet:  I  observe  also  that  the  planet  Mars,  and 


148  ASTRONOMY- 

consequently  the  otlier  superior  planets,  are  much 
nearer  the  earth  at  one  time  than  at  others. 

r.  The  difference  with  respect  to  Mars  is  no  less 
than  190  millions  of  miles,  the  whole  length  of  the 
orbit  of  the  earth.  This  will  be  a  proper  time  to  ex- 
plain what  is  meant  by  the  heliocentric  longitude  of 
the  planets  referred  to  in  the  Ephemeris. 

J.  Yes,  I  remember  you  promised  to  explain  this 
when  you  came  to  speak  of  the  planets :  I  do  not 
know  the  meaning  of  the  word  heliocentric. 

T.  It  is  a  term  used  to  express  the  place  of  any 
heavenly  body  as  seen  from  the  sun  ;  whereas  the 
geocentric  place  of  a  planet,  is  the  position  which  it 
has  when  seen  from  the  earth. 

C.  Will  you  shew  us  by  a  figure  in  what  this  dif- 
ference consists  1 

T,  I  will :  let  s  repre- 
sent the  place  of  the  sun, 
b  Venus  in  its  orbit,  a 
the  earth  in  hers,  and  c 
Mars  in  his  orbit,  and 
the  outermost  circle  will 
represent  the  sphere  of 
fixed  stars.  Now  to  a 
spectator  on  the  earth, 
a,  Venus  will  appear 
among  the  fixed  stars  in 
the  beginning  of  Scorpio,  Fig.  19. 

but  as  viewed  from  the 

sun,  she  will  be  seen  beyond  the  middle  of  Leo. 
Therefore  the  geocentric  longitude  of  Venus  will  be 
in  Scorpio,  but  her  heliocentric  longitude  will  be  in 
Leo.  Again,  to  a  spectator  at  a,  the  planet  Mars  at 
c  will  appear  among  the  fixed  stars  towards  the  end 
of  the  sign  of  Pisces  ;  but,  as  viewed  from  the  sun, 
he  will  be  seen  at  the  beginning  of  the  sign  Aries  : 
consequently,  the  geocentric  longitude  of  Mars  is  in 
Pisces,  but  his  heliocentric  longitude  is  in  Aries. 


OF  JUPITER. 


149 


CONVERSATION  XXI. 

OF  JUPITER. 

T.  We  now  come  to  Jupiter,  the  largest  of  all  the 
planets,  which  is  easily  known  by  his  peculiar  mag- 
nitude and  brilliancy  c 

C.  Is  Jupiter  larger  than  Venus  1 

T.  Though  he  does  not  appear  so  large,  yet  the 
magnitude  of  Venus  bears  but  a  very  small  proportion 
to  that  of  Jupiter,  whose  diameter  is  90,000  miles  in 
length,  consequently  his  bulk  will  exceed  the  bulk 
of  Venus  1500  times:  his  distance  from  the  sun  is 
estimated  at  more  than  490  millions  of  miles. 

J.  Then  he  is  Jive  times  farther  from  the  sun  than 
the  earth  ;  and,  consequently,  as  light  and  heat  dimi- 
nish in  the  same  proportion  as  the  squares  of  the 
distances  from  the  illuminating  body  increase,  the  in- 
habitants of  Jupiter  enjoy  but  a  twenty-fifth  part  of 
the  light  and  heat  of  the  sun  that  we  enjoy. 

T.  Another  thing  remarkable  in  this  planet  is,  that 
it  revolves  on  its  axis,  which  is  perpendicular  to  its 
orbit,  in  10  hours,  and  in  consequence  of  this  swift 
diurnal  rotation,  his  equatorial  diameter  is  6000  miles 
greater  than  his  polar  diameter. 

C.  Since,  then,  a  variety  in  the  seasons  of  a  planet 
depends  upon  the  inclination  of  the  axis  to  its  orbit, 
and  since  the  axis  of  Jupiter  has  no  inclination,  there 
can  be  no  difference  in  his  seasons,  nor  any  in  the 
length  of  his  days  and  nights. 

r.  You  are  right ;  his  days  and  nights  are  always 
five  hours  each  in  length ;  and  at  his  equator,  and  its 
neighbourhood,  there  is  a  perpetual  summer ;  and  an 
everlasting  winter  in  the  polar  regions. 

J.  What  is  the  length  of  his  years  1 

T.  It  is  equal  to  nearly  12  of  ours,  for  he  takes  11 
years,  314  days,  and  10  hours,  to  make  a  revolution 
round  the  sun,  consequently  he  travels  at  the  rate  of 
more  than  28,000  miles  in  an  hour. 


150 


ASTRONOMY. 


This  noble  planet  is  accompanied  with  four  satel- 
lites, which  were  first  discovered  by  Galileo  in  1610  ; 
they  revolve  about  him  at  different  distances,  and  in 
different  periodical  times ;  the  first  in  about  1  day 
and  18  hours :  the  second  in  3  days  13  hours :  the 
third  in  7  days  3  hours :  and  the  fourth  in  16  days 
and  16  hours. 

C.  And  are  these  satellites,  like  our  moon,  subject 
to  be  eclipsed  ? 

T,  They  are ;  and  these  eclipses  are  of  consider- 
able importance  to  astronomers,  in  ascertaining  with 
accuracy  the  longitude  of  different  places  on  the 
earth. 

By  means  of  the  eclipses  of  Jupiter's  satellites,  a 
method  has  been  also  obtained  of  demonstrating  that 
the  motion  of  light  is  progressive,  and  not  instantane- 
ous, as  was  once  supposed.  Hence  it  is  found,  that 
the  velocity  of  light  is  nearly  11,000  times  greater 
than  the  velocity  of  the  earth  in  its  orbit,  and  more 
than  a  million  of  times  greater  that  that  of  a  ball 
issuing  from  a  cannon.  Kays  of  light  come  from  the 
sun  to  the  earth  in  8  minutes,  that  is,  at  the  rate  of 
about  12  millions  of  miles  in  a  minute. 


CONVERSATION  XXII. 

OF  SATURN. 

T.  We  are  now  arrived  at  Saturn  in  our  descrip- 
tions, which,  till  within  these  fifty  years,  was  es- 
teemed the  most  remote  planet  of  the  solar  system. 

C.  How  is  he  distinguished  in  the  heavens  ? 

T.  He  shines  with  a  pale  dead  light,  very  unlike 
the  brilliant  Jupiter,  yet  his  magnitude  seems  to  vie 
with  that  of  Jupiter  himself.  The  diameter  of  Saturn 
is  nearly  80  thousand  miles  in  length  :  his  distance 
from  the  sun  is  more  than  900  millions  of  miles,  and 
lie  performs  his  journey  round  that  luminary  in  a 
little  less  than  30  of  our  years,  consequently  he  must 


OF  SATURN.  151 
travel  at  a  rate  not  much  short  of  21,000  miles  an 
hour. 

J.  His  great  distance  from  the  sun  must  render 
an  abode  on  Saturn  extremely  cold  and  dark  too,  in 
comparison  of  what  we  experience  here. 

r.  His  distance  from  the  sun  being  between  nine 
and  ten  times  greater  than  that  of  the  earth,  he  must 
enjoy  about  90  times  less  light  and  heat;  it  has 
nevertheless  been  calculated  that  the  light  of  the  sun 
at  Saturn  is  500  times  greater  than  what  we  enjoy 
from  OMYfiili  moon. 

C.  The  daylight  at  Saturn,  then,  cannot  be  very 
contemptible  :  I  should  hardly  have  thought  that  the 
light  of'the  sun  here  was  500  times  greater  tlmn  that 
experienced  from  a  full  moon. 

T.  So  much  greater  is  our  meridian  light  than  this, 
that  during  the  sun's  absence  behind  a  cloud,  when 
the  light  is  much  less  strong  than  when  we  behold 
him  in  all  his  glorious  splendour,  it  is  reckoned  that 
our  daylight  is  90,000  times  greater  than  the  light  of 
the  moon  at  its  full. 

J.  But  Saturn  has  several  moons,  I  believe  1 

T.  He  is  attended  by  seven  satellites,  or  moons, 
whose  periodical  times  differ  very  much  ;  the  one 
nearest  to  him  performs  a  revolution  round  the  pri- 
mary planet  in  22  hours  and  a  half ;  and  that  which 
is  most  remote  takes  79  days  and  7  hours  for  his 
monthly  journey  :  this  last  satellite  is  known  to  turn 
on  its  axis,  and  in  its  rotation  is  subject  to  the  same 
law  which  our  moon  obeys,  that  is,  it  revolves  on  its 
axis  in  the  same  time  in  which  it  revolves  about  the 
planet. 

Besides  the  seven  moons,  Saturn  is  encompassed 
with  two  broad  rings,  which  are  probably  of  con- 
siderable importance  in  reflecting  the  light  of  the  sun 
to  that  planet :  the  breadth  of  the  inner  ring  is  20,000 
miles,  that  of  the  outer  ring  7200  miles,^and  the  va- 
cant space  between  the  two  rings  is  2839  miles. 
These  rings  give  Saturn  a  very  different  appearance 


152 


ASTRONOMY. 


to  any  of  the  other  pla- 
nets. This  figure  is  a 
representation  of  Sa- 
turn, as  seen  through 
a  good  telescope. 

C.  Does  Saturn  turn 
on  its  axis  1 

T.  According  to  Dr. 
Herschel  it  has  a  rota- 
tion about  its  axis  in  12  hours  13|  minutes :  this  he 
computed  from  the  equatorial  diameter  being  longer 
than  the  polar  diameter  in  the  proportion  of  11  to  10. 
Dr.  Herschel  has  also  discovered,  that  the  ring,  ju&t 
mentioned,  revolves  about  the  planet  in  10  hours  and 
a  J.  He  also  considers  that  the  ring  is  no  less  solid 
than  the  body  of  the  planet  itself ;  and  has  observed 
that  it  casts  a  shadow  upon  the  planet,  and  that  the 
light  of  the  ring  is  brighter  than  that  of  the  planet. 

CONVERSATION  XXIII. 

OF  THE   HERSCHEL  PLANET, 

T.  We  have  but  the  planet  Herschel  now  to  describe. 

J.  Was  it  discovered  by  Dr.  Herschel  ? 

T.  It  was,  on  the  13th  of  March,  1781  ;  and 
therefore  by  many  astronomers  it  is  denominated  the 
Herschel  planet ;  though  by  the  Doctor  himself  it 
was  named  the  Georgium  Sidus,  or  Georgian  Star,  in 
honour  of  his  late  Majesty  George  the  Third,  who 
was  for  many  years  a  liberal  patron  to  this  great  and 
most  indefatigable  astronomer  :  but  by  foreign  astro- 
nomers it  is  usually  called  Uranus. 

C.  I  do  not  think  that  I  have  ever  seen  this  planet. 

T.  Its  apparent  diameter  is  too  small  to  be  dis- 
cerned readily  by  tlie  naked  eye,  but  it  may  be  easily 
discovered  in  a  clear  night,  when  it  is  above  the  hori- 
zon, by  means  of  a  good  telescope,  its  situation  being 
previously  known  from  the  Ephemeris. 

J.  Is  it  owing  to  the  smallness  of  this  planet,  or  to 
its  great  distance  from  the  sun,  that  we  cannot  see  it 
with  the  naked  eye  1 


OF  THE  HERSCHEL  PtANET.  153 

T.  Both  these  causes  are  combined :  in  compari- 
son of  Jupiter  and  Saturn  it  is  small,  his  diameter 
being  less  than  35  thousand  miles  in  length  ;  and  his 
distance  from  the  sun  is  estimated  at  more  than  1800 
millions  of  miles  from  that  luminary,  around  which, 
however,  he  performs  his  journey  in  84  of  our  years, 
consequently  he  must  travel  at  the  rate  of  16,000 
miles  an  hour. 

C.  But  if  this  planet  has  been  discovered  only  52 
years,  how  is  it  known  that  it  will  complete  its  revo- 
tion  in  84  years? 

T.  By  a  long  series  of  observations,  it  was  found 
to  move  with  such  a  velocity  as  would  carry  it  round 
the  heavens  in  that  period.  And  it  has  been  ascer- 
tained, that  all  the  computations  of  its  places,  con- 
ducted upon  that  supposition,  are  correct. 

J.  How  many  moons  has  the  Herschel  ? 

T.  He  is  attended  by  six  satellites  or  moons,  of 
which,  the  one  nearest  to  the  planet  performs  his 
revolution  round  the  primary  in  5  days  and  25  hours, 
but  that  which  is  the  most  remote  from  him  takes  107 
days  and  16  hours  for  his  journey. 

C.  Is  there  any  idea  formed  as  to  the  light  and 
heat  enjoyed  by  this  planet  1 

T.  His  distance  from  the  sun  is  19  times  greater 
than  that  of  the  earth ;  consequently,  since  the  square 
of  19  is  361,  the  light  and  heat  experienced  by  the 
inhabitants  of  that  planet  must  be  361  times  less 
than  ive  derive  from  the  rays  of  the  sun. 

The  proportion  of  light  enjoyed  by  the  Herschel 
has  been  estimated  at  about  equal  to  the  effect  of  248 
of  our  full  moons. 

The  following  Synopsis  presents,  at  one  view,  the 
periods,  distances,  and  magnitudes,  of  all  the  planets^ 
including  the  four  small  ones  lately  discovered : — 


112 


354 


ASTRONOMY. 


Eartl 

1 — 

'eal 

greatei 

ba 

times 

•tenth 

and  3- 

o 
o 

CO 

r— 1 

o 

[ETS, 
1,448; 

^    ^    ^     toD    bb  bo 


CO     CO  O) 


!-<  <M 


O      JO  suoi|{Tui  ui  ung  9qj  luoij  83iii;iSTp  irL)9i\[ 


o 

>^  .  „  _ 

02  (:Ncococ^r-ic<t(M 


C<J         OH  (  ^  ^ 

-  t^TfO-M^i-HOf-^^COCO 

C0(MCOC^<MCO(NCN— ICO 


 \'  

UI  uoT:in|OA9'y;  oipoii9jj 

 ^.A  


5"   rt  ^ 
HH    O  y:  ^ 


OF  COMETS. 


155 


CONVERSATION  XXIV. 

OF  COMETS. 

r.  Besides  the  seven  primary  planets,  and  the 
eighteen  secondary  ones,  or  satellites,  which  we  have 
been  describing,  there  are  other  bodies  belonging  to 
the  solar  system,  called  comets. 

C.  Do  comets  resemble  the  planets  in  any  respects  ? 

T.  Like  them  they  are  supposed  to  revolve  about 
the  sun  in  elliptical  orbits,  and  to  describe  equal  areas 
in  equal  times  ;  but  they  do  not  appear  to  be  adapted 
for  the  habitation  of  animated  beings,  owing  to  the 
great  degrees  of  heat  and  cold  to  which,  in  their 
course,  they  must  be  subjected. 

The  comet  seen  by  Sir  Isaac  Newton  in  the  year 
1680,  was  observed  to  approach  so  near  the  sun,  that 
its  heat  was  estimated  by  that  great  man  to  be  2000 
times  greater  than  that  of  red-hot  iron. 

J.  It  must  have  been  a  very  solid  body  to  have  en- 
dured such  a  heat  without  being  entirely  dissipated. 

T,  So  indeed  it  should  seem  :  and  a  body  thus 
heated  must  retain  its  heat  a  long  time  ;  for  a  red-hot 
globe  of  iron  of  a  single  inch  in  diameter,  exposed  to 
the  open  air,  will  scarcely  lose  all  its  heat  in  an  hour ; 
and  it  is  said,  that  a  globe  of  red-hot  iron  as  large  as 
our  earth,  would  scarcely  cool  in  50,000  years. 

C.  Are  the  periodical  times  of  the  comets  known  1 

T.  Not  with  any  degree  of  certainty  ;  it  was  sup- 
posed that  the  periods  of  three  of  them  had  been  dis- 
tinctly ascertained.  The  first  of  these  appeared  in 
the  years  1531,  1607,  and  1682,  and  it  v/as  expected 
to  return  every  75th  year;  and  one  which,  as  Dr. 
H alley  had  predicted,  appeared  in  1758,  was  sup- 
posed to  be  the  same. 

The  second  of  them  appeared  in  1532,  and  1661, 
and  it  was  expected  that  it  would  again  make  its  ap- 
pearance in  1789,  but  in  this  the  astronomers  have 
been  disappointed. 


15G 


ASTRONOMY. 


The  third  was  that  which  appeared  in  1680,  and 
its  period  being  estimated  at  575  years  cannot,  upon 
that  supposition,  return  until  the  year  2255.  This 
last  comet  at  its  greatest  distance  is  eleven  thousand 
two  hundred  millions  of  miles  from  the  sun,  and  its 
least  distance  from  the  sun's  centre  was  but  four 
hundred  and  ninety  thousand  miles  ;  in  this  part  of 
its  orbit  it  travelled  at  the  rate  of  880,000  miles  in 
an  hour. 

J.  Do  all  bodies  move  faster  or  slower  in  propor- 
tion as  they  are  nearer  to,  or  more  distant  from,  their 
centre  of  motion  ? 

T.  They  do,  for  if  you  look  back  upon  the  last  six 
or  seven  lectures,  you  will  see  that  the  Herschel, 
which  is  the  most  remote  planet  in  the  solar  system, 
travels  at  the  rate  of  16,000  miles  an  hour  ;  Saturn 
the  next  nearer  in  the  order  21,000  miles  ;  Jupiter 
28,000  miles  ;  Mars  53,000  miles  ;  the  earth  65,000 
miles ;  Venus  75,000  miles  ;  and  Mercury  at  the  rate 
of  105,000  miles  in  an  hour.  But  here  we  come  to  a 
comet,  whose  progressive  motion,  in  that  part  of  its 
orbit  which  is  nearest  to  the  sun,  is  more  than  equal 
to  eight  times  the  velocity  of  Mercury. 

C.  Were  not  comets  formerly  dreaded,  as  awful 
prodigies  intended  to  alarm  the  world  ? 

T.  Comets  are  frequently  accompanied  with  a  lu- 
minous train  called  the  tail,  which  is  supposed  to  be 
nothing  more  than  smoke  rising  from  the  body  in  a 
line  opposite  to  the  sun,  but  which,  to  uninformed 
people,  has  been  a  source  of  terror  and  dismay. 

I'wo  very  brilliant  comets  have  made  their  appear- 
ance within  the  last  twenty -seven  years.  One  of  them 
was  seen  for  several  weeks  in  1807,  and  the  other 
from  September  to  the  end  of  1811.  Dr.  Herschel 
has  given  an  elaborate  account  of  the  first  in  the  98th 
vol.  of  the  Philosophical  Transactions;  the  tail  of 
which  was  ascertained  to  be  more  than  nine  millions 
of  miles  in  length  ;  and  the  tail  of  that  which  appeared 
in  1811  was  full  33  millions  in  length. 


OF  THE  FIXED  STARS. 


157 


CONVERSATION  XXV. 

OF  THE  SUN. 

T.  Having  given  you  a  particular  description  of 
the  planets  vi^hich  revolve  about  the  sun,  and  also  of 
the  satellites  which  travel  round  the  primary  planets 
as  central  bodies,  while  they  are  carried  at  the  same 
time  with  these  bodies  round  the  sun,  we  shall  con- 
clude our  account  of  the  solar  system  by  taking  some 
notice  of  the  sun  himself. 

J.  You  told  us  a  few  days  ago  that  the  sun  has  a 
rotation  on  its  axis  ;  how  is  that  known  ? 

T.  By  the  spots  on  his  surface  it  is  known  that  he 
completes  a  revolution  from  west  to  east  on  his  axis 
in  about  25  days,  two  days  less  than  his  apparejit 
revolution,  in  consequence  of  the  earth's  motion  in 
her  orbit,  in  the  same  direction. 

C.  Is  the  figure  of  the  sun  globular  ? 

T.  No the  motion  about  its  axis  renders  it  sphe- 
roidical, having  its  diameter  at  the  equator  longer 
than  that  which  passes  through  the  poles. 

The  sun's  diameter  is  more  than  equal  to  100  diame- 
ters of  the  earth,  and  therefore  his  bulk  must  be  more 
than  a  million  of  times  greater  than  that  of  the  earth, 
but  the  density  of  the  matter  of  which  it  is  composed 
is  four  times  less  than  the  density  of  our  globe. 

We  have  already  seen  that  by  the  attraction  of  the 
sun  the  planets  are  retained  in  their  orbits,  and  that 
to  him  they  are  indebted  for  light,  heat,  and  motion. 


CONVERSATION  XXVI. 

OF  THE  FIXED  STARS. 

T.  We  will  now  put  an  end  to  our  astronomical 
conversations  by  referring  again  to  the  fixed  stars, 
which  like  our  sun  shine  by  their  own  light. 

C.  Is  it  then  certain  that  the  fixed  stars  are  of 


158  ASTRONOMY. 

themselves  luminous  bodies  ;  and  that  the  planets 

borrow  their  light  from  the  sun  ? 

T.  By  the  help  of  telescopes  it  is  known  that  Mer- 
cury, Venus,  and  Mars,  shine  by  a  borrowed  light, 
for,  like  the  moon,  they  are  observed  to  have  different 
phases  according  as  they  are  differently  situated  with 
regard  to  the  sun.  The  immense  distances  of  Jupiter, 
Saturn,  and  the  Herschel  planet,  do  not  allow  the 
difference  between  the  perfect  and  imperfect  illumi- 
nation of  their  discs  or  phases  to  be  perceptible. 

Now  the  distance  of  the  fixed  stars  from  the  earth 
is  so  great,  that  reflected  light  would  be  much  too 
weak  ever  to  reach  the  eye  of  an  observer  here. 

J.  Is  this  distance  ascertained  with  any  degree  of 
precision  ? 

T.  It  is  not :  but  it  is  known  with  certainty  to  be 
so  great,  that  the  whole  length  of  the  earth's  orbit, 
viz.  190  millions  of  miles,  is  but  a  point  in  comparison 
of  it ;  and  hence  it  is  inferred,  that  the  distance  of  the 
nearest  fixed  star  cannot  be  less  than  a  hundred 
thousand  times  the  length  of  the  earth's  orbit ;  that 
is,  a  hundred  thousand  times  190  millions  of  miles, 
or  19,000,000,000,000  miles  :  this  distance  being  im- 
mensely great,  the  best  method  of  forming  some  clear 
conception  of  it,  is  to  compare  it  with  the  velocity  of 
some  moving  body,  by  which  it  may  be  measured. 
The  swiftest  motion  with  which  we  are  acquainted  is 
that  of  light,  which  as  we  have  seen  is  at  the  rate  of 
12  millions  of  miles  in  a  muiute  ;  and  yet  light 
would  be  about  3  years  in  passing  from  the  nearest 
fixed  star  to  the  earth. 

A  cannon-ball,  which  may  be  made  to  move  at  the 
rate  of  20  miles  in  a  minute,  would  be  1800  thousand 
years  in  traversing  this  distance.  Sound,  the  velocity 
of  which  is  13  miles  in  a  minute,  would  be  more  than 
two  millions  seven  hundred  thousand  years  in  passing 
from  the  star  to  the  earth.  So  that  if  it  were  possible 
for  the  inhabitants  of  the  earth  to  see  the  light,  to  hear 
the  sound,  and  to  receive  the  ball,  of  a  cannon  dis- 
charged at  the  nearest  fixed  star,  they  would  not  per- 


OF  THE  FIXED  STARS.  15\^ 

ceive  the  light  of  its  explosion  for  three  years  after  it 
had  been  fired,  nor  receive  the  ball  till  1800  thousand 
years  had  elapsed,  nor  hear  the  report  for  two  mil- 
lions and  700  hundred  thousand  years  after  the  ex- 
plosion. 

C.  Are  the  fixed  stars  at  difiPerent  distances  from 
the  earth  1 

T.  Their  magnitudes,  as  you  know,  appear  to  be 
different  from  one  another,  which  diflference  may  arise 
either  from  a  diversity  of  their  real  magnitudes,  or  in 
their  distances,  or  from  both  these  causes  acting  con- 
jointly. It  is  the  opinion  of  Dr.  Herschel  that  the  dif- 
ferent apparent  magnitudes  of  the  stars  arise  from  the 
difiPerent  distances  at  which  they  are  situated,  and 
therefore  he  concludes  that  stars  of  the  seventh  mag- 
nitude are  at  seven  times  the  distance  from  us  that 
those  of  the  first  magnitude  are. 

By  the  assistance  of  his  telescopes  he  was  able  to 
discover  stars  at  497  times  the  distance  of  Sirius,  the 
Dogstar :  from  which  he  inferred,  that  with  more 
powerful  instruments  he  should  be  able  to  discover 
stars  at  still  greater  distances. 

J.  I  recollect  that  you  told  us  once,  that  it  had 
been  supposed  by  some  astronomers,  that  there  might 
be  fixed  stars  at  so  great  a  distance  from  us,  that  the 
rays  of  their  light  had  not  yet  reached  the  earth, 
though  they  had  been  travelling  at  the  rate  of  12 
millions  of  miles  in  a  minute  from  the  first  creation  to 
the  present  time. 

r,  I  did ;  it  was  one  of  the  sublime  speculations 
of  the  celebrated  Huygens.  Dr.  Halley  has  also  ad- 
vanced what,  he  says,  seems  to  be  a  metaphysical 
paradox,  viz.  that  the  number  of  fixed  stars  must  be 
more  than  finite,  and  some  of  them  at  a  greater  than 
a  finite  distance  from  others  :  and  Mr.  Addison  has 
justly  observed,  that  this  thought  is  far  from  being 
extravagant,  when  we  consider  that  the  universe  is  the 
work  of  infinite  power,  prompted  by  infinite  goodness, 
and  having  an  infinite  space  to  exert  itself  in :  so 
that  our  imagination  can  set  no  bounds  to  it. 


ASTRONOMY. 

C.  "What  can  be  the  use  of  those  fixed  stars  not 
to  enlighten  the  earth,  for  a  single  additional  moon 
would °give  us  much  more  light  than  them  all,  espe- 
cially if  it  were  so  contrived  as  to  afford  us  its  assis- 
tance at  those  intervals  when  our  present  moon  is  be- 
low the  horizon. 

T.  You  are  right :  they  could  not  have  been  cre- 
ated for  our  use,  "since  thousands,  and  even  millions, 
are  never  seen  but  by  the  assistance  of  glasses,  to 
which  but  few  of  our  race  have  access.  Your  mmds 
indeed  are  too  enlightened  to  imagine,  like  chil- 
dren unaccustomed  to  reflection,  that  all  things  were 
created  for  the  enjoyment  of  man.  The  earth  on 
which  we  live  is  but  one  of  seven  primary  planets 
circulating  perpetually  round  the  sun  as  a  centre,  and 
with  these  are  connected  eighteen  secondary  planets  or 
moons  all  of  which  are  probably  teeming  with  livmg 
beings  capable,  though  in  different  ways,  of  enjoying 
the  bounties  of  the  great  First  Cause. 

.The  fixed  stars  then  are  probably  suns,  which, 
like  our  sun,  serve  to  enlighten,  warm,  and  sustain 
other  systems  of  planets  and  their  dependant  satel- 
lites. P    ,  ^ 

J.  Would  our  sun  appear  as  a  fixed  star  at  any 

mat  distance  1  ,  -r^     tt      u  wt,-  i  . 

T  It  certainly  would  ;  and  Dr.  Herschel  thinks 
there  is  no  doubt,  but  that  it  is  one  of  the  heavenly 
bodies  belonging  to  that  tract  of  the  heavens  known 
by  the  name  of  the  Mi%  IFai/. 

CI  know  the  rnilky  way  in  the  heavens,  but  i 
little  thought  that  I  had  any  concern  with  it  other- 
wise than  as  an  observer. 

T  The  milky  way  consists  of  fixed  stars  too  small 
to  be  discerned  with  the  naked  eye  ;  and  if  our  sun 
be  one  of  them,  the  earth  and  other  planets  are  closely 
connected  with  this  part  of  the  heavens. 

But  P-entlemen,  it  is  time  that  we  take  our  leave 
of  this 'subject  for  the  present.  For  your  attention  to 
those  instructions  which  on  this  and  other  topics  I  have 
been  able  to  communicate,  accept  my  best  thanks. 


OF  THE  FIXED  STARS, 


161 


For  your  future  welfare  and  happiness  my  heart  is 
deeply  interested.  You  will  not,  I  flatter  myself,  very 
soon  forget  that  connexion  which  has  subsisted  be- 
1  tween  us  for  a  long  course  of  years.    From  my  mind 
the  remembrance  of  your  kindness  can  never  be  ob- 
'  liter ated.    Permit  me,  then,  as  a  testimony  of  my 
j  gratitude  and  sincere  affection,  to  recommend  to  your 
j  future  attention  the  works  of  nature  and  creation,  by 
a  careful  investigation  of  which  you  will  be  necessa- 
rily led  to  the  contemplation  and  love  of  the  God  of 
Nature. 

Your  knowledge,  young  as  you  yet  are,  of  the  fun- 
i  damental  principles  of  Geometry  and  Algebra,  is 
j  such  as  to  render  scientific  pursuits  easy  and  pleasant. 
And  your  understandings  are  not  more  capable  of 
entering  into  the  sublime  speculations  of  science, 
than  your  hearts  are  adapted  to  receive  and  cherish 
those  impressions  of  gratitude,  which  are  the  natural 
consequences  of  enlarged  and  comprehensive  views 
of  the  being  and  perfections  of  the  Deity. 


HYDROSTATICS. 


CONVERSATION  I. 
INTRODUCTION. 

FATHER  CHARLES — EMMA. 

Father.  In  pursuing  our  course  of  natural  and  ex- 
perimental philosophy,  we  shall  now  proceed  with 
that  branch  of  science  which  is  called  Hydrostatics. 

Emma.  That  is  a  difficult  word :  what  are  we  to 
understand  by  it  1 

F.  Almost  all  the  technical  terms  made  use  of  in 
science  are  either  Greek,  or  derived  from  the  Greek 
language.  The  word  hydrostatics  is  formed  of  two 
Greek  words,  which  signify  icater,  and  the  science 
which  considers  the  weight  of  bodies.  But  hydrosta- 
tics, as  a  branch  of  natural  philosophy,  treats  of  the 
nature,  gravity,  pressure,  and  motion  of  fluids  in 
general ;  and  of  the  methods  of  weighing  solids  in 
them. 

Charles.  Is  this  an  important  part  of  knowledge  ? 

F,  Taken  in  this  extensive  sense,  it  yields  to  none 
as  to  its  real  importance.  And  the  experiments  which 
I  shall  shew  you  are  curious,  and  highly  amusing. 

E.  Shall  we  be  able  to  repeat  them  ourselves  ? 

F.  Most  of  them  you  will,  provided  you  are  very 
careful  in  using  the  instruments,  almost  all  of  which 
are  made  of  glass.  I  ought  to  tell  you  that  many 
writers  divide  this  subject  into  two  distinct  parts,  viz. 
hydrostatics  and  hydrauiics ;  the  latter  relates  parti- 
cularly to  the  motion  of  water  through  pipes,  con- 
duits, &c. 

In  these  Conversations  I  shall  pay  no  regard  to 
this  distinction,  but  shall,  under  the  general  title  of 
hydrostatics,  describe  the  properties  of  all  fluids,  but 
principally  those  of  water ;  explaining  as  we  go  on 


THE  DEFINITION  OF  FLUIDS.  163 

the  motions  of  it,  whether  in  pipes,  pumps,  syphons, 
engines  of  different  kinds,  fountains,  &c.  Do  you 
know  what  a  fluid  is  ? 

C.  I  know  how  to  distinguish  a  fluid  from  a  solid  : 
water  and  wine  are  fluids;  but  why  they  are  so 
called  I  cannot  tell. 

!  F.  A  fluid  is  generally  defined  as  a  body,  the  parts 
of  which  readily  yield  to  any  impression,  and  in 

I  yielding  are  easily  moved  amongst  each  other. 

E.  But  this  definition  does  not  notice  the  wetting 
of  other  bodies  brought  into  contact  with  a  fluid.    If  I 

|!  put  my  fingers  into  water  or  milk,  a  part  of  it  adheres 

'  to  them,  and  they  are  said  to  be  wet. 

j  F.  Every  accurate  definition  must  mark  the  quali- 
ties of  all  the  individual  things  defined  by  it :  now 
there  are  many  fluids  which  have  not  the  property  of 
wetting  the  hand  when  plunged  into  them.    The  air 

j  we  breathe  is  a  fluid,  the  parts  of  which  yield  to  the 

I  least  pressure,  but  it  does  not  adhere  to  the  bodies 

j  surrounded  by  it  like  water. 

I  E.  Air,  however,  is  so  different  from  water,  that, 
I  in  this  respect,  they  will  scarcely  admit  of  com- 
parison. 

I  C.  I  have  sometimes  dipped  my  finger  into  a  cup 
1  of  quicksilver,  but  none  of  the  fluid  came  away  with  it. 
!  F,  You  are  right ;  and  hence  you  will  find  that 
(natural  philosophy  distinguishes  between  fluids  and 
j  liquids.  Air,  quicksilver,  and  melted  metals,  are 
I  fluids,  but  not  liquids  :  while  water,  milk,  beer,  wine, 
oil,  spirits,  &c.  are  fluids  and  liquids. 
I  C.  Are  we  then  to  understand,  that  liquids  are 
'  known  by  the  property  of  adhering  to  diflferent  sub- 
!  stances  which  are  immersed  in  them  ? 
j  F.  This  description  will  not  always  hold ;  for 
though  mercury  will  not  stick  to  your  hand  if 
plunged  into  a  cup  of  it,  yet  it  will  adhere  to  many 
■metals,  as  tin,  gold,  &c.  And  therefore  you  will  re- 
member, that  the  distinction  between  liquids  and 
'fluids  is  introduced  into  books  more  on  account  of 
common  convenien'*e  th^n  ^h'M^oT^Hy,^al  accuracy. 

I 


1G4  HYDROSTATICS. 

E.  You  said,  I  believe,  that  a  fluid  is  defined  as  a 
body,  whose  parts  yield  to  the  smallest  force  im- 
pressed; 

F.  This  is  the  definition  of  a  perfect  fluid  ;  and  the 
less  force  that  is  required  to  move  the  parts  of  a  fluid, 
the  more  perfect  is  that  fluid. 

C.  But  how  do  people  reason  respecting  the  parti- 
cles of  which  fluids  are  composed?  have  they  ever 
seen  them  1 

F.  Philosophers  imagine  they  must  be  exceed mgly 
small,  because  with  their  best  glasses  they  have  never 
been  able  to  discern  them.  And  they  contend  that 
these  particles  must  be  round  and  smooth,  since  they 
are  so  easily  moved  among  and  over  one  another.  If 
they  are  round,  you  know  there  must  be  vacant  spaces 
between  them. 

E.  How  is  that,  papa  ? 

F.  Suppose  a  number  of  cannon  balls 
were  placed  in  a  large  tub,  or  any  other 
vessel,  so  as  to  fill  it  up  even  with  the  edge  : 
though  the  vessel  would  contain  no  more  of 
these  large  balls,  yet  it  would  hold  in  the 
vacant  spaces  a  great  many  smaller  shot; 
and  between  these  again  others  still  smaller  Fig.  1 . 
might  be  introduced  ;  and  when  the  barrel 

would  contain  no  more  small  shot,  a  great  quantity  of 
sand  might  be  shaken  in,  between  the  pores  of  which 
water  or  other  fluids  would  readily  insinuate  them- 
selves. 

E.  This  I  understand;  but  are  there  any  other 
proofs  that  water  is  made  up  of  such  globular  particles '? 

F.  There  are  several : — all  aquatic  plants,  that  is, 
plants  which  live  in  water,  are  said  to  have  their  pores 
round,  and  are  thereby  adapted  to  receive  the  same 
shaped  particles  of  water :  all  mineral  and  medicinal 
waters  evidently  derive  their  peculiar  character  from 
the  different  substances  taken  into  their  pores  ;  from 
which  it  has  been  concluded,  that  the  particles  of 
water  are  globular,  because  such  admit  of  the  largest 
intervals. 


OF  THE  PARTICLES  OF  FLUIDS.  165 

Upon  this  principle  tinctures,  as  those  of  bark, 
rhubarb,  &c.  are  made  :  a  quantity  of  the  powder  of 

I  bark,  or  any  other  substance,  is  put  into  spirits  of 

I  wine  ;  the  very  fine  particles  are  taken  into  the  pores 
of  the  spirit ;  these  change  the  colour  of  the  mass, 

5  though  it  remains  as  transparent  as  it  was  before. 

j     C.  But  in  these  cases  is  not  the  bulk  of  the  fluid 

!  increased  ? 

F.  In  some  instances  it  is  ;  but  in  others  the  bulk 
will  remain  precisely  the  same,  as  the  following  very 
easy  experiment  will  shew  : — 

i  Take  a  phial  with  some  rain-water,  mark  very  ac- 
j  Curately  the  height  at  which  the  water  stands  in  the 
bottle  ;  after  which  you  may  introduce  a  small  quan- 
tity of  salt,  which,  when  completely  dissolved,  you 
;will  find  has  not  in  the  least  increased  the  bulk  of  the 
water  When  the  salt  is  taken  up,  still  sugar  may 
be  dissolved  in  the  water  without  making  any  addi- 
jtion  to  its  bulk. 

;  E.  Are  we  then  to  infer,  that  the  particles  of  salt 
iare  smaller  than  those  of  water,  and  lie  between 
|them,  as  the  small  shot  lie  between  the  cannon-balls, 
jand  that  the  particles  of  sugar  are  finer  than  those  of 
,salt,  and,  like  the  sand  among  the  shot,  will  insinuate 
(themselves  into  vacuities  too  small  for  the  admission 
iof  the  salt  I 

\  F.  I  think  the  experiment  fairly  leads  to  that  con- 
Iclusion.  Another  fact  respecting  the  particles  of 
i'fluids  deserving  your  notice  is,  that  they  are  exceed- 
ingly hard,  and  almost  incapable  of  compression. 

C.  What  do  you  mean,  sir,  by  compression  ? 
1  F.  I  mean  the  act  of  squeezing  any  thing,  in  order 
to  bring  its  parts  nearer  together.  Almost  all  sub- 
Istances  with  which  we  are  acquainted  may,  by  means 
of  pressure,  be  reduced  into  a  less  space  than  they  na- 
turally occupy.  But  water,  oil,  spirits,  quicksilver, 
l&c.  cannot,  by  any  pressure,  of  which  human  art  or 
ipower  is  capable,  be  reduced  into  a  space  sensibly 
I  less  than  they  naturally  possess. 

E.  Has  the  trial  ever  been  made  ? 

i 


106 


HYDROSTATICS. 


F.  Ye>;,  by  some  of  the  ablest  philosophers  that 
ever  lived.  And  it  has  been  found  that  water  v/ill 
find  its  v^^ay  through  the  pores  of  gold,  rather  than 
suffer  itself  to  be  compressed  into  a  smaller  space. 

C.  How  did  that  happen  ? 

F,  At  Florence,  a  celebrated  city  in  Italy,  a  globe 
made  of  gold  wdiS  filled  with  water,  and  then  closed 
so  accurately  that  none  of  it  could  escape.  The  globe 
was  then  put  into  a  press,  and  a  little  flattened  at  the 
sides  :  the  consequence  of  which  was,  that  the  w^ater 
came  through  the  fine  pores  of  the  golden  globe,  and 
stood  upon  its  surface  like  drops  of  dew. 

C.  Would  not  the  globe,  then,  contain  so  much 
after  its  sides  were  bent  in  as  it  did  before  1 

F.  It  would  not ;  and  as  the  water  forced  its  way 
through  the  gold,  rather  than  suffer  itself  to  be 
brought  into  a  smaller  space  than  it  naturally  occu- 
pied, it  was  concluded  at  that  time  that  water  was  in- 
compressible. Later  experiments  have,  however, 
shewn,  that  those  fluids  which  v/ere  esteemed  incom- 
pressible are,  in  a  very  small  degree,  as,  perhaps,  one 
part  in  twenty  thousand,  capable  of  compression. 

E.  Is  it  on  this  account  you  conclude  that  the  par- 
ticles are  very  hard  ? 

F.  Undoubtedly  ;  for,  if  they  were  not  so,  you  can 
easily  conceive,  that  since  there  are  vacuities  between 
them,  as  we  have  shewn,  and  as  are  represented  in 
Fig.  1.  they  must  by  very  great  pressure  be  brought 
closer  together,  and  vi^ould  evidently  occupy  a  less 
space,  which  is  contrary  to  fact. 


CONVERSATION  IT. 

OF  THE  WEIGHT  AND  PRESSURE  OF  FLUIDS. 

F.  In  our  last  conversation  we  considered  the 
nature  of  the  component  parts  of  fluids  :  I  must  now 
tell  you,  that  these  parts  or  particles  act,  with  respect 
to  their  weight  or  pressure  independently  of  each 
other. 


OF  THE  PRESSURE  OF  FLUIDS.  167 

E.  Will  you  explain  what  you  mean  by  this  ? 

F,  You  recollect,  that  by  the  attraction  of  cohesion* 
the  parts  of  all  solid  substances  are  kept  together,  and 
press  into  one  common  mass.  If  I  cut  a  part  of  this 
wooden  ruler  away,  the  rest  will  remain  in  precisely 
the  same  situation  as  it  was  before.  But  if  1  take 
[some  water  out  of  the  middle  of  a  vessel,  the  remain- 
der flows  instantly  into  the  place  from  whence  that 
was  taken,  so  as  to  bring  the  whole  mass  to  a  level. 

C.  Have  the  particles  of  water  no  attraction  for 
each  other  1  » 

F.  Yes,  in  a  slight  degree.  The  globules  of  dewf 
on  cabbage  plants  prove,  that  the  particles  of  water 
have  a  greater  attraction  to  one  another,  than  they 
have  to  the  leaf  on  which  they  stand.  Nevertheless, 
ithis  attraction  is  very  small,  and  you  can  easily  con- 
jCeive,  that  if  the  particles  are  round,  they  will  touch 
|each  other  in  very  few  parts,  and  slide  with  the 
smallest  pressure.  Imagine  that  a  few  of  the  little 
{globules  were  taken  out  of  the  vessel,  (Fig.  1.)  and 
jit  is  evident  that  the  surrounding  ones  would  fall  into 
•their  place.  It  is  upon  this  principle  that  the  surface 
iof  every  fluid,  when  at  rest,  is  horizontal  or  level. 

C.  Is  it  upon  this  principle  that  water  levels  are 
i  constructed  'i 

F.  It  is  :  the  most  simple  kind  of  water  level  is  a 
long  wooden  trough,  which  being  filled  to  a  certain 
height  with  water,  its  surface  shews  the  level  of  the 
place  on  which  it  stands. 

J  C.  I  did  not  allude  to  this  kind  of  levels,  but  to 
those  smaller  ones  contained  in  glass  tubes. 

I    F.  These  are,  more  properly  speaking,  air-levels. 

[They  are  thus  constructed  :  d  is  a     „  ^ 
glass  tube  fixed  into  l,  a  socket  made  C^^^^^S") 
generally  of  brass.    The  glass  is 
filled  with  water,  or  some  other  fluid.        Fig.  2. 
in  which  is  enclosed  a  single  bubble 

*  See  Mechanics,  Conver.  III. 
t  See  Mechanics,  Conver.  IV. 


1€8  HYDROSTATICS, 
of  air.    When  this  bubble  fixes  itself  at  the  mark  a, 
made  exactly  in  the  middle  of  the  tube,  the  place  on 
which  the  mstrument  stands  is  perfectly  level.  When 
It  IS  not  level,  the  bubble  will  rise  to  the  higher  end. 

E,  What  is  the  use  of  these  levels  ? 

F.  They  are  fixed  to  a  variety  of  philosophical  in- 
slruments,  such  as  quadrants,  and  telescopes  for  sur- 
veymg  the  heavens,  and  theodolites  for  takino-  the 
level  of  any  part  of  the  earth.  They  are  also  itseful 
in  the  more  common  occurrences  of  life.  A  sino^le 
mstance  will  shew  their  value :  clocks  will  not  ke'ep 
true  time  unless  they  stand  very  upright ;  now  by 
means  of  one  of  these  levels  you  may  easily  ascer- 
tam  whether  the  bracket,  upon  which  the  clock  in  the 
passage  stands,  is  level. 

E,  But  I  remember  when  Mr.  F  brouo-ht 

home  your  clock,  he  tried  if  the  bracket  was  even  "by 
means  of  one  of  Charles's  marbles.  How  did  he 
know  by  this  ? 

F,  The  marble,  being  round,  touched  the  board 
m  a  point  only,  consequently  the  line  of  direction* 
could  not  fall  through  that  point,  but  the  marble 
would  roll,  unless  the  bracket  was  very  level ;  there- 
fore, when  the  marble  was  placed  in  two  or  more 
different  parts  of  the  board,  and  did  not  move  to  one 
side  or  the  other,  he  might  safely  conclude  that  it 
was  level. 

C.  Then  the  water-level  and  the  rolling  of  the 
marble  depend  on  the  same  principle. 

F.  They  do,  upon  the  supposition  that  the  particles 
of  water  are  round.  The  water-level  will,  however^ 
be  the  most  accurate,  because  we  may  imagine  that 
the  parts  of  which  water  is  composed  are  perfectly 
round,  and,  therefore,  as  may  be  geometricallv  proved, 
they  will  touch  only  in  an  infinitely  small  point ; 
whereas,  marbles  made  by  human  contrivance  touch 
in  many  such  points. 

We  now  come  to  another  very  curious  principle  in 

*  See  Mechanics,  Coaver.  IX. 


OF  THE  UPWARD  PRESSURE.  1G9 

tills  branch  of  science,  viz.  that  fluids  press  equally  in 
all  directions.  All  bodies,  both  fluid  and  solid,  press 
liownwards  by  the  force  of  gravitation,  but  fluids  of 
all  kinds  exert  a  pressure  upwards  and  sideways 
equal  to  their  pressure  downwards. 

-E.  Can  you  shew  any  experiments  in  proof  of  this  ? 

F.  a,  b,  c,  is  a  bended  glass 
fube  :  with  a  small  glass  funnel 
pour  into  the  mouth  a  a  quan- 
:ity  of  sand.  You  will  find 
.hat,  when  the  bottom  part  is 
illed,  whatever  is  poured  in 
ifterwards  will  stand  in  the  side  Fig.  3,  Fig.  4. 
Df  the  tube  a  6,  and  not  rise  m 
Lhe  other  side  b  c. 

C.  The  reason  of  this  is,  that  by  the  attraction  of 
g'ravitation  all  bodies  have  a  tendency  to  the  earth,* 
hat  is,  in  this  case,  to  the  lowest  part  of  the  tube ; 
3ut,  if  the  sand  ascended  in  the  side  b  c,  its  motion 
jVould  be  directly  the  reverse  of  this  principle. 

F.  You  mean  to  say  that  the  pressure  would,  be 
ipwards,  or  from  the  centre  of  the  earth. 

C.  It  certainly  would. 
|i  F.  Well,  we  will  pour  away  the  sand,  and  put 
ijvater  in  its  place  :  what  do  you  say  to  this  ? 
I   E.  The  water  is  level  in  both  sides  of  the  tube, 
i  F.  This  then  proves,  that  with  respect  to  fluid&y 
[here  is  a  pressure  upwards  at  the  point  b  as  well  as 
lownwards.    I  will  shew  you  another  experiment. 

A  B  is  a  large  tube  or  jar  having  a  flat  bot-  « 
om  :  a  6  is  a  smaller  tube  open  at  both  ends.  Jk 
While  I  fill  the  jar  with  water,  I  take  care 
0  hold  the  small  tube  so  close  to  the  bottom 
)f  the  jar  as  to  prevent  any  water  from  get- 
ing  into  the  tube.    I  then  raise  it  a  little, 
ind  you  see  it  is  instantly  filled  with  water  ^ 
Tomthejar. 


*  See  Mechanics,  Conver,  Y. 
1 


170 


HYDROSTATICS. 


C.  It  is;  and  the  water  is  level  in  the  jar  and  tube. 

F.  The  tube  you  saw  was  filled  by  means  of  the 
pressure  upwards,  contrary  to  its  natural  gravity. 

Take  out  the  tube  ;  now  the  water  having  escaped, 
it  is  filled  with  air.  Stop  the  upper  end  a  with  a  cork, 
and  plunge  it  into  the  jar,  the  water  will  only  rise  as 
high  as  h, 

E.  What  is  the  reason  of  this,  papa  ? 

F.  The  air  with  which  the  tube  was  filled  is  a  body, 
and  unless  the  water  were  first  to  force  it  out  from 
the  tube,  it  cannot  take  its  place.  While  this  ink- 
stand remains  here,  you  are  not  able  to  put  any  other 
thing  in  the  same  part  of  space. 

C.  If  air  be  a  substance,  and  the  tube  is  filled  with 
it,  how  can  any  water  make  its  way  into  the  tube  I 

F.  This  is  a  very  proper  question.  Air,  though  a 
substance,  and,  as  we  have  already  observed,  a  fluid 
too,  differs  from  water  in  this  respect,  that  it  is  easily 
compressible,  that  is,  the  air,  wiiich,  by  the  natural 
pressure  of  the  surrounding  atmosphere,  fills  the  tube, 
may,  by  the  additional  upward  pressure  of  the  water, 
be  reduced  into  a  smaller  space,  as  a  b.  Another 
experiment  will  illustrate  the  difference  between  com- 
pressible and  incompressible  fluids. 

Fill  the  tube,  which  has  still  a  cork  in  one  end, 
with  some  coloured  liquor,  as  spirits  of  wine  ;  over 
the  other  end  place  a  piece  of  pasteboard,  held  close 
to  the  tube,  to  prevent  any  of  the  liquor  from  escap- 
ing :  in  this  way  introduce  the  tube  into  a  vessel  of 
water,  keeping  it  perpendicular  all  the  time :  you  may 
now  take  away  the  pasteboard,  and  force  the  tube 
to  any  depth,  but  the  spirit  of  wine  is  not  like  the 
air,  it  cannot  in  this  manner  be  reduced  into  a  space 
smaller  than  it  originally  occupied. 

E.  Why  did  not  the  spirits  of  wine  run  out  of  the 
tube  into  the  water  ? 

F,  Because  spirits  are  lighter  than  water,  and  it  is 
a  general  principle  that  tiie  lighter  fluid  always  as- 
cends to  the  top. 

Take  a  thin  piece  of  horn  or  pasteboard,  and  while 


OF  THE  PRESSURE  OF  FLUIDS.  17i 

you  hold  it  by  the  edges,  let  your  brother  put  a 
i  pound  weight  upon  it :  what  is  the  result  ? 

E.  It  is  almost  bent  out  of  my  hand. 

i    F.  Introduce  it  now  into  a  vessel  of  water  at  the 
depth  of  twelve,  or  fifteen  inches,  and  bring  it  parallel 
to  the  surface.    In  this  position,  it  sustains  many 
pounds'  weight  of  water. 
I    C.  Nevertheless,  it  is  not  bent  in  the  least. 

F.  Because  the  upward  pressure  against  the  lower 
jsurface  of  the  horn  is  exactly  equal  to  the  pressure 
down  vizard,  or,  which  is  the  same  thing,  it  is  equal  to 
the  weight  of  the  water  which  it  sustains  on  the  upper 
surface  ;  in  other  words,  fluids  press  equally  in  all 
[directions." 

You  may  vary  these  experiments  by  yourselves  till 
we  meet  again  j  when  we  shall  resume  the  same 
Isubject. 

I  CONVERSATION  III. 

I  OF  THE  WEIGHT  AND  PRESSURE  OF  FLUIDS. 

i  C.  When  you  were  explaining  the  principle  of  the 
iWheel  and  Axis,*'  I  asked  the  reason  why,  as  the 
^bucket  ascended  near  the  top  of  the  well,  the  difficulty 
iiin  raising  it  increased?  1  have  just  now  found  another 
ipart  of  the  subject  beyond  my  comprehension.  After 
;the  bucket  is  hlled  with  water,  it  sinks  to  the  bottom 
{of  the  well,  or  as  far  as  the  rope  will  suffer  it ;  but  in 
jdrawing  it  up  through  the  water,  it  seems  to  have 
■little  or  no  weight  till  it  has  ascended  to  the  surface 
iOf  the  water.  How  is  this  accounted  for  1 
i  F.  1  do  not  wonder  that  you  have  noticed  this  cir- 
icumstance  as  singular.  It  was  long  believed  by  the 
I  ancients  that  water  did  not  gravitate,  or  had  no  weight 
'in  water :  or,  as  they  used  to  express  it  more  generally, 
ithat  fluids  do  not  gravitate  in  propria  loco. 
1  E.  I  do  not  understand  the  meaning  of  these  hard 
I  words. 

i  *  See  Mechanics,  Conver.  XVII. 


172  HYDROSTATICS. 

F,  Nor  would  I  have  made  use  of  them,  only  that 
you  can  scarcely  open  a  treatise  on  this  subject  with- 
out finding  the  phrase.  I  will  explain  their  meaning 
without  translating  the  words,  because  a  mere  trans- 
lation would  give  you  a  very  inadequate  idea  of  what 
the  writers  intended  to  express  by  them. 

No  one  ever  doubted  that  water  and  other  fluids 
had  weight  when  considered  by  themselves  ;  but  it 
was  supposed  that  they  had  no  weight  when  immersed 
in  a  fluid  of  the  same  kind.  The  fact  which  your 
brother  has  just  mentioned  respecting  the  bucket  was 
the  grand  argument  upon  which  they  advanced  and 
maintained  this  doctrine. 

E.  Does  it  not  weigh  any  thing,  then,  till  it  is 
drawn  above  the  surface  ? 

F.  You  must,  my  little  girl, 
have  patience,  and  you  shall  see 
how  it  is.  Here  is  a  glass  bottle 
A,  with  a  stop-cock  b  cemented  to 
it,  by  means  of  which  the  air  may 
be  exhausted  from  the  bottle,  and 
prevented  from  returning  into  it 
again.  The  whole  is  made  suf- 
ficiently heavy  to  sink  in  the  vessel  of  water  c  d  . 

The  bottle  must  be  weighed  in  air,  that  is,  in  the 
common  method,  and  suppose  it  weighs  12  ounces ; 
let  it  now  be  put  into  the  situation  which  is  represented 
by  the  figure,  when  the  weight  of  the  bottle  must  be 
again  taken  by  putting  weights  into  the  scale  z.  I 
then  open  the  stop-cock  while  it  is  under  water,  and 
the  water  immediately  rushes  in  and  fills  the  bottle, 
which  overpowers  the  weights  in  the  scale.  I  now 
put  other  weights,  say  8  ounces,  into  the  scale,  to 
restore  the  equilibrium  between  the  bottle  and  scale 
It  is  evident,  then,  that  8  ounces  is  the  weight  of  th 
water  in  the  bottle,  while  weighed  under  water 
Fasten  the  cock  and  weigh  the  bottle  in  the  usu 
way  in  the  air. 

C.  It  weighs  something  more  than  20  ounces. 

F.  That  is  12  ounces  for  the  bottle,  and  8  ounc 


OF  THE  PRESSURE  OF  FLUIDS. 


173 


lor  the  water,  besides  a  small  allowance  to  be  made 
for  the  drops  of  water  that  adhere  to  the  outside  of  the 
bottle.  Does  not  this  experiment  prove  that  the 
water  in  the  bottle  weighed  just  as  much  in  the  jar 
of  water,  as  it  weighed  in  the  air  ? 
jE.  I  think  it  does. 

F.  Then  we  are  justified  in  concluding  that  the 
water  in  the  bucket,  which  the  bottle  may  represent, 
weighed  as  much  while  under  water  in  the  well,  as  it 
did  after  it  was  raised  above  the  surface. 

C.  This  fact  seems  decisive,  but  the  difficulty  still 
remains  in  my  mind,  for  the  weight  of  the  bucket  is 
not  felt  till  it  is  rising  above  the  surface  of  the  water. 

F.  It  may  be  thus  accounted  for  :  any  substance 
of  the  sam.e  specific  gravity  with  water,  may  be 
plunged  into  it,  and  it  will  remain  wherever  it  is 
placed,  either  near  the  bottom,  in  the  middle,  or 
towards  the  top,  consequently  it  may  be  moved  in 
any  direction  with  the  application  of  a  very  small 
force. 

E.  What  do  you  mean  by  the  specific  gravity  of  a 
body  ? 

!  F.  The  specific  gravity  of  any  body  is  its  weight 
compared  with  that  of  any  other  body.  Hence  it  is 
also  called  the  comparative  gravity  :  thus,  if  a  cubic 
inch  of  water  be  equal  in  weight  to  a  cubic  inch  of 

I  any  particular  kind  of  wood,  the  specific  or  compara- 
tive gravities  of  the  water  and  that  wood  are  equal. 
But,  since  a  cubic  inch  of  deal  is  lighter  than  a 
cubic  inch  of  water,  and  that  is  lighter  than  the  same 
bulk  of  lead  or  brass,  we  say  the  specific  gravity  of 
the  lead,  or  brass,  is  greater  than  that  of  water,  and 
the  specific  gravity  of  water  greater  than  that  of  deal. 

C.  The  water  in  the  bucket  must  be  of  the  same 
specific  gravity  with  that  in  the  well,  because  it  is  a 
part  of  it. 

jP.  And  the  wooden  bucket  differs  very  little  in 
this  respect  from  the  water  ;  because,  though  the 
wood  is  lighter,  yet  the  iron  of  which  the  hoops  and 
handle  are  composed  is  specifically  heavier  than 


174  HYDROSTATICS, 
water  ;  so  that  the  bucket  and  water  are  nearly  of  the 
same  specific  gravity  with  the  water  in  the  well,  and 
therefore  it  is  moved  very  easily  through  it. 

Again,  we  have  already  proved  that  the  upward 
pressure  of  fluids  is  equal  to  the  pressure  downwards, 
therefore  the  pressure  at  the  bottom  of  the  bucket 
upwards  being  precisely  equal  to  the  same  force  in  a 
contrary  direction,  the  application  of  a  very  small 
force,  in  addition  to  the  upward  pressure,  will  cause 
the  bucket  to  ascend. 

E.  Do  you  account  for  the  easy  ascent  of  the 
bucket  upon  the  same  principle  by  which  you  have 
shewn  that  horn  or  pasteboard  will  not  be  bent,  when 
placed  horizontally  at  any  depth  in  water  1 

F,  Yes,  I  do  :  and  I  will  shew  you  some  other 
experiments  to  prove  the  effect  of  the  upward  pressure. 

Take  a  glass  tube,  open  at  both  ends,  the  diameter 
of  which  is  about  the  eighth  of  an  inch,  fill  it  with 
water,  and  close  the  top  with  your  thumb  ;  you  may 
now  take  it  out  of  the  water,  but  it  will  not  empty 
itself  so  long  as  the  top  is  kept  closed. 

C.  This  is  not  the  upward  pressure  of  water,  be- 
cause the  tube  was  taken  out  of  it. 

F.  You  are  right :  it  is  the  upward  pressure  of  the 
air,  which,  while  the  thum.b  is  kept  on  the  top,  is  not 
counterbalanced  by  any  downward  pressure,  and 
therefore  it  keeps  the  water  suspended  in  the  tube. 

Take  this  ale-glass  and  fill  it  with  water,  and 
cover  it  with  a  piece  of  writing-paper :  then  place 
your  hand  evenly  over  the  paper,  so  as  to  hold  it  very 
tight  about  the  edge  of  the  glass,  which  you  may  now 
invert,  and  take  away  your  hand  without  any  danger 
of  the  water  falling  out. 

E.  Is  the  water  sustained  by  the  upward  pressure 
of  the  air  1 

F.  The  upward  pressure  of  the  air  against  the 
paper  sustains  the  weight  of  water  and  prevents  it 
from  falling 

You  have  seen  the  instrument  used  for  tasting  of 
beer  or  wine  ? 


OF  THE  PRESSURE  OE  FLUIDS.  1T5 

E.  Yes  :  it  is  a  tin  tube,  that  holds  about  half  a 
i^int^  into  which  very  small  tubes  are  inserted  at  the 
iitop  and  bottom.  ,    ,  ,        i  r 

F.  The  longer  one  is  put  into  the  hole  made  tor 
the  vent-peg,  and  then  the  beer  or  wine  is,  by  draw- 
incr  out  the  air  from  it,  forced  into  the  large  part  of 
the  tube,  and  by  putting  the  thumb  or  finger  on  the 
upper  part,  the  whole  instrument  may  be  taken  out  ot 
the  cask,  and  removed  any  where,  for  the  pressure  of 
the  air  against  the  bottom  surface  of  the  lower  tube 
keeps  the  liquor  from  running  out,  but  the  moment 
the  thumb  is  taken  from  the  top,  the  liquor  descends 
i,by  the  downward  pressure  of  the  air. 

C.  Is  it  for  a  similar  reason  that  vent-holes  are 
made  in  casks  1 

F.  It  is  :  for  when  a  cask  is  full,  there  is  no  down- 
:  ward  pressure,  and  therefore  the  air  pressing  against 
'  the  mouth  of  the  cock  keeps  the  liquor  from  runnmg 
ilout ;  a  hole  made  at  the  top  of  the  cask  admits  the 
;  external  pressure  of  the  air,  by  which  the  liquor  is 
'  forced  out.  In  large  casks  of  ale  or  porter,  where 
;  the  demand  is  not  very  great,  the  vent-hole  need 
i  seldom  be  used,  for  a  certain  portion  of  the  air  con- 
tained in  the  liquor  escapes,  and  being  lighter  than 
I  the  beer,  ascends  to  the  top,  by  which  a  pressure  is 
'  created  without  the  assistance  of  the  external  air. 


CONVERSATION  IV. 

OF  THE   LATERAL   PRESSURE  OF  FLUIDS. 

F.  It  is  time  now  to  advance  another  step  in  this 
science,  and  to  shew  you  that  the  lateral  or  side  pres- 
sure is  equal  to  the  perpendicular  pressure. 

E.  If  the  upward  pressure  is  equal  to  the  down- 
ward, and  the  side  pressure  is  also  equal  to  it,  then  the 
pressure  is  equal  in  all  directions. 

F.  You  are  right.  Though  the  side  direction  may 
be  varied  in  many  ways,  yet  there  are  only  the  up- 


HYDROSTATICS, 
ward,  downward,  and  lateral  directions.    The  two 
lormer  we  have  shewn  are  equal.  That  the  side  pres- 
sure  is  equal  to  the  perpendicular  pressure  downwards 
IS  demonstrated  by  a  very  easy  experiment. 

AB  IS  a  vessel  filled  with  water,  havino-  a~ 
two  equal  orifices  or  holes,  ah,  bored  with  the 
same  tool,  one  at  the  side,  and  the  other  in  c 
the  bottom  ;  if  these  holes  are  opened  at  the 
same  instant,  and  the  water  suffered  to  run  -MJ^ 
into  two  glasses,  it  will  be  found  that,  at  the  x^/  '*  ^ 
end  of  a  given  time,  they  will  have  discharged  ^' 
equal  quantities  of  water  ;  which  is  a  clea^r  proof  that 
w^rds^^^""  P'^^^^s  side-wise  as  forcibly  as  it  does  down- 

*u  ^^^"^      ^^^^         a  general  principle 

that  fluids  press  in  every  possible  direction  ? 

F.  This  I  think  our  experiments  have  proved  .  but 
you  must  not  forget  that  it  is  only  true  upon  the  sup- 
position that  the  perpendicular  heights  are  equal.  For 
m  the  last  experiment,  if  the  hole  h  had  been  bored 
an  inch  or  two  higher  in  the  side  of  the  vessel,  as  at 
c,  the  quantity  of  water  running  out  at  a  would  have 
been  greater  than  that  at  h,  and  much  greater  would 
It  have  been  if  the  hole  had  been  bored  at  four  or 
hve  inches  above  the  bottom  of  the  vessel 

This  subject  of  pressure  may  be  farther  illustrated. 
A.t  the  bottom  of  this  tube  nv,  open  at  both 
ends,  1  have  tied  a  piece  of  bladder,  and 
have  poured  in  water  till  it  stands  at  the 
mark  r.   Owing  to  the  pressure  of  the  water, 
the  bladder  is  convex,  that  is,  bent  outwards  •  ' 
dip  It  mto  thejar,  (Fig,  5.  p.  169.)  theblad'  . 
der  is  stdl  convex  :  thrust  it  gently  down, 
the  surface  of  the  water  in  the  tube  is  now 
even  with  that  in  thejar. 
^^E.  It  is  ;  and  the  bladder  at  the  bottom  is  become 

F.  The  perpendicular  depths  being  equal  the 
pressure  upwards  is  equal  to  that  downwards,  and  the 


OF  THE  PRESSURE  OF  FLUIDS.  m 
water  in  the  tube  is  exactly  balanced  by  the  water  in 
the  jar.  Let  the  tube  be  thrust  deeper  into  tlie 
water. 

C.  Now  the  bladder  is  bent  upwards. 

F.  The  upward  pressure  is  estimated  by  the  per- 
pendicular depth  of  the  water  in  the  jar,  measured 
from  the  surface  to  the  bottom  of  the  tube  ;  but  the 
pressure  downwards  must  be  estimated  by  the  per- 
pendicular height  of  the  water  in  the  tube,  which 
being  less  than  the  former,  the  pressure  upwards  m 
the  same  proportion  overcomes  that  downwards,  and 
forces  up  the  bladder  into  the  position  as  you  see  it. 
This  and  the  following  experiment  are  some  of  the 
best  that  can  be  exhibited  in  proof  of  the  upward 
pressure  of  fluids. 

Dip  an  open  end  of  a  tube,  having  a  very  narrow 
bore,  into  a  vessel  of  quicksilver ;  then  stopping  the 
upper  orifice  with  the  finger,  lift  up  the  tube  out  of 
the  vessel,  and  you  will  see  a  sort  of  column  of  quick- 
silver hanging  at  the  lower  end,  which,  when  dipped 
in  water  lower  than  fourteen  times  its  own  length, 
will,  upon  removing  the  finger,  be  pressed  upwards 
into  the  tube. 

E.  Why  do  you  fix  upon  14  times  the  depth  ? 

F.  Because  quicksilver  is  14  times  heavier  than 
water.  Upon  this  principle  of  the  upward  pressure, 
lead  or  any  other  metal  may  be  made  to  swim  in 
water,  ab  is  a  vessel  of  water,  and  ab  is  a 
glass  tube  open  throughout,  d  is  string  by  "^j^j^' 
which  a  flat  piece  of  lead  x  may  be  held  ""^ — ^ 
fast  to  the  bottom  of  the  tube.  To  prevent 
the  water  from  getting  in  between  the  lead 
and  the  glass,  a  piece  of  wet  leather  is  first 
put  over  the  lead. 

In  this  situation,  let  the  tube  be  immersed 
in  the  vessel  of  water,  and  if  it  be  plunged  to  the 
depth  of  about  eleven  times  the  thickness  of  the  lead 
before  the  string  be  let  go,  the  lead  will  not  fall  from 
the  tube,  but  be  kept  adhering  to  it  by  the  upward 
pressure  below  it. 

I  2 


178 


HYDROSTATICS. 


E.  Is  lead  11  times  heavier  than  water? 

F.  It  is  between  11  and  12  times  heavier  ;  and 
therefore  to  make  the  experiment  sure,  the  tube  should 
be  plunged  somewhat  deeper  than  11  times  the  thick- 
ness of  the  lead. 

C.  Is  it  not  owing  to  the  wet  leather  that  the  lead 
sticks  to  the  tube,  rather  than  to  the  upward  pres- 
sure ? 

F.  If  that  be  the  case,  it  will  remain  fixed  if  I 
draw  up  the  tube  an  inch  or  two  higher  : — I  will 
try  it. 

E.  It  has  fallen  off. 

F.  Because,  when  the  tube  was  raised,  the  up- 
ward pressure  was  diminished  so  much  as  to  become 
too  small  to  balance  the  weight  of  the  lead.  But  if 
the  adhering  together  of  the  lead  and  tube  had  been 
caused  by  the  leather,  there  would  be  no  reason  why 
it  should  not  operate  the  same  at  six  or  nine  times  the 
depth  of  the  lead's  thickness  as  well  as  at  11  or 
12  times  that  thickness. 


CONVERSATION  V. 

OF   THE   IIYDROSTATICAL  PARADOX. 

E.  You  are  to  explain  a  paradox  to-day  :  I  thought 
natural  philosophy  had  excluded  all  paradoxes. 

F.  Dr.  Johnson  has  given  this  definition  of  a  para- 
dox, an  assertion  contrary  to  appearances  ;"  now 
the  assertion  to  which  I  am  to  refer  you  is,  that  any 
Qiuutiilii  of  water,  however  small,  may  be  made  to  ba- 
UDice  and  support  any  quantity,  hoicever  lari^e.  That 
a  pound  of  water,  for  instance,  should,  without  any 
mechanical  advantage,  be  made  to  support  ten  pounds, 
or  a  hundred,  or  even  a  ton  weight,  seems  at  first  in- 
credible ;  certainly  it  is  contrary  to  what  one  should 
expect,  and  on  that  account  the  experiment  to  shew 
this  fact  has  usually  been  called  the  hydrostatical 
paradox. 


HYDROSTATIC  PARADOX. 


179 


C.  It  does  appear  unaccountable  :  I  hope  the  ex- 
periments may  be  very  easy  to  be  understood. 

F.  Many  have  been  invented  for  the  purpose,  but 
I  know  of  none  better  than  those  described  by  Mr. 
Ferguson  in  his  Lectures  on  Select  Subjects. 

OEGH  is  a  glass  vessel,  consisting  of  two  tubes  of 
very  different  sizes,  joined  together,  and   ^  ^ 
freely  communicating  with  one  another.  ^  J  // 
Let  water  be  poured  in  at  h,  which  will     H  ' 
pass  through  the  joining  of  the  tubes,  and  "7/ 
rise  in  the  wide  one  to  the  same  height  ex-       /  ^' 
actly  as  it  stands  in  the  smaller ;  which 
shews  that  the  small  column  of  water  in  -p-^ 
DG  balances  the  large  one  in  the  other  tube.  °' 
This  will  be  the  case  if  the  quantity  of  water  in  the 
small  tube  be  a  thousand  or  a  million  of  times  less 
than  the  quantity  in  the  larger  one. 

If  the  smaller  tube  be  bent  in  any  oblique  situa- 
tion, as  GF,  the  water  v/ill  stand  at  f,  that  is,  on  the 
same  level  as  it  stands  at  a.  This  would  be  the  case, 
if  instead  of  two  tubes  there  were  any  given  number 
of  them  connected  together  at  b,  and  varied  in  all 
kinds  of  oblique  directions,  the  water  would  be  on 
a  level  in  them  all;  that  is,  the  iper-pendicidur  height 
of  the  water  would  be  the  same. 

C.  This  does  not  quite  satisfy  me,  because  it  ap- 
pears that  a  great  part  of  the  water  in  the  large  tube 
is  supported  by  the  parts  b  about  the  bottom,  and 
therefore  that  the  water  in  the  smaller  tube  only  sus- 
tains the  pressure  of  a  column  of  water,  the  diameter 
of  which  is  equal  to  its  own  diameter. 

-F.  This  would  be  the  case  if  the  pressure  of  fluids 
was  only  downwards,  but  we  have  shewn  that  it  acts 
in  all  directions.  And  therefore  the  pressure  of  the 
parts  near  the  side  of  the  tube  acts  against  the  column 
in  the  middle,  which  you  suppose  is  the  only  part  of 
the  water  sustained  by  that  which  is  contained  in  the 
small  tube;  consequently  the  smaller  quantity  of 
v/ater  in  db  sustains  the  larger  one  in  ab. 


180 


HYDROSTATICS. 


Fi^.  11 


Let  us  try  another  experiment. 

AB  and  AB  are  two  ves- 
sels, having  their  bottoms 
■Dd  and  i>d  exactly  equal, 
but  the  contents  of  one 
vessel  is  20  times  greater 
than  the  other ;  that  is, 
Fig.  11,  when  filled  up  to 
A,  will  hold  but  one  pint 
of  v/ater,  whereas  Fig.  12..  when  filled  to  the  same 
height,  will  hold  20  pints.  Brass  bottoms,  cc,  are 
fitted  exactly  to  each  vessel,  and  made  water  tight  by 
pieces  of  wet  leather.  Each  bottom  is  joined  to  its 
vessel  by  a  hinge  d,  so  that  it  opens  downwards, 
like  the  lid  of  a  box.  By  means  of  a  little  hook  d, 
a  pulley  f,  and  a  weight  e,  the  bottom  is  kept  close 
to  the  vessel,  and  will  hold  a  certain  quantity  of  water. 

E.  That  is,  till  the  weight  of  the  water  overcome 
the  weight  e. 

F.  I  should  rather  say,  till  the  pressure  of  the  water 
overcome  the  weight  e. 

Now  hold  the  vessel  (Fig.  12.)  upright  in  your 
hands,  while  I  gradually  pour  water  into  it  with  a 
funnel  ;  the  pressure  bears  down  the  bottom,  and,  of 
course,  raises  the  weight,  and  a  small  quantity  of  the 
water  escapes.  Let  us  mark  the  height  h,  at  which 
the  surface  of  the  water  stood  in  the  vessel  when  the 
bottom  began  to  give  way. 

Try  the  other  vessel  (Fig.  11.)  in  the  same  manner, 
and  we  shall  see  that  when  the  water  rises  to  a,  that 
is,  to  just  the  same  height  m  this  vessel  as  in  the  for- 
mer, the  bottom  will  also  give  way,  as  it  did  in  the 
other  case.  Thus  equal  weights  are  overcome  in  the 
one  case  by  20  pints  of  water,  and  in  the  other  by  a 
single  pint.  The  same  would  hold  good  if  tlie  dif- 
ference was  greater  or  less  in  any  given  proportion. 

E.  What  is  the  reason  of  this,  papa  ? 

F.  It  depends  upon  two  principles  with  which  you 
arc  acquainted.    The  first  is,  that  fluids  press  equally 


HYDROSTATIC  PARADOX.  181 


m  all  directions  ;  and  the  second  is,  that  action  and 
re-action  are  equal  and  contrary  to  each  other.*  The 
water,  therefore,  below  the  fixed  part  Bgf  will  press  as 
much  upward  against  the  inner  surface,  by  the  ac- 
tion of  the  small  column  Ag,  as  it  would  by  a  column 
of  the  sinne  height,  and  of  any  other  diameter  whatso- 
ever :  and  since  action  and  re-action  are  equal  and 
contrary,  the  action  against  the  inner  surface  Bo-y  will 
cause  an  equal  re-action  of  the  water  in  the  cavity 
b/"cd  against  the  bottom  c ;  consequently  the  pressure 
upon  cc,  Fig.  11.,  will  be  as  great  as  it  was  upon  the 
same  part  of  Fig.  12. 

C.  Can  you  prove  by  experiment  that  there  is  this 
upward  pressure  against  the  inner  surface  Bg  f  ? 

F.  Very  easily  :  suppose  at  f  there  were  a  little 
cork,  to  which  a  small  string  was  fixed :  I  might 
place  a  tube  over  the  cork,  and  then  draw  it  out,  the 
consequence  of  which  would  be,  that  the  water  in  the 
vessel  would  force  itself  into  the  tube,  and  stand  as 
high  in  it  as  it  does  in  the  vessel.  Would  not  this 
experiment  prove  that  there  was  this  upward  pres- 
sure against  Bg/? 

C.  It  would  :  and  I  can  easily  conceive,  that  if 
other  tubes  were  placed  in  the  same  manner,  in  dif- 
ferent parts  of  Bgf\  the  same  effect  would  be  pro- 
duced. 

F,  Then  you  must  admit,  that  the  action  against 
Eg-/,  or,  which  is  the  same  thing,  the  re-action  against 
c,  that  is,  the  pressure  of  the  water  against  the  bot- 
tom, is  equally  great  as  it  would  be  if  the  vessel  were 
as  large  in  every  part  as  it  is  at  the  bottom,  and  the 
water  stood  level  to  the  height  ^a. 

C.  Yes,  I  do  :  because  if  tubes  were  placed  in 
every  part  of  b/,  the  same  eflPect  would  be  produced 
ia  them  all,  as  in  the  single  one  at  /  ,-  but  if  the 
whole  surface  were  covered  with  small  tubes,  there 
would  then  be  little  or  no  diflference  between  the  two 
vessels.  Figs.  11.  and  12. 


*  See  Mechanics,  Conver.  XL 


182  HYDROSTATICS. 

F.  There  would  be  no  difference,  provided  you 
kept  filling  the  large  tube,  so  that  the  water  should 
stand  in  them  all  at  the  same  level  Aa.  Otherwise, 
the  introduction  of  a  single  tube  o/ would  make  a 
material  difference.  For  though  the  water  in  ac 
vv'ould  overcome  the  weight  e,  yet  if  with  my  hand  I 
prevent  any  of  the  water  from  running  out  till  I  have 
taken  out  the  cork,  and  suffered  the  water  to  force 
itself  out  of  the  vessel  into  the  small  tube,  I  may  re- 
move my  hand  with  safety  ;  for  the  water  will  not 
overcome  the  weight  now,  though  there  is  certainly 
the  same  quantity  of  water  in  it  as  there  was  before 
the  little  tube  a/ was  inserted. 

E.  I  think  I  see  the  reason  of  this  :  the  water  stood 
as  high  as  Aa  before  the  little  tube  was  introduced,  but 
now  it  stands  at  the  level  xx,  and  you  told  us  yester- 
day that  the  pressures  were  only  equal,  provided  the 
perpendicular  heights  were  also  equal. 

F.  I  am  glad  to  find  you  so  attentive  to  what  I 
say.  In  order  that  the  pressure  may  overcome  the 
weight  E,  you  must  put  in  more  water  till  it  rise  to 
the  level  Aa,  and  now  you  see  the  weight  rises,  and 
the  water  flows  out. 

I  will  now  put  another  tube  at  ^,  and  the  water 
rushing  into  that  causes  the  level  to  descend  again  to 
XX,  and  I  must  put  more  water  in  to  bring  the  level 
up  to  A,  before  it  can  overcome  the  weight  e.  What 
I  have  shewn  in  these  two  cases  will  hold  true  in  all, 
supposing  you  fill  the  cover  with  tubes. 

C.  I  see,  then,  that  it  is  the  difference  of  the  per- 
pendicular heights  which  causes  the  difference  of 
pressure,  and  can  now  fully  comprehend  the  reason 
why  a  pint  of  water  may  be  made  to  balance  or  sup- 
port a  hogshead  :  or,  in  the  words  with  which  you 
set  out,  that  any  quantity  of  water,  however  small,  may 
he  made  to  halance  and  support  any  other  quantity,  how- 
ever larcre. 

F.  What  has  been  proved  respecting  water  holds 
with  regard  to  wine,  oil,  or  any  other  fluid  :  but  not 
if  diifcront  fluids  are  used  together,  as  water  and  oil. 


HYDROSTATIC  BELLOWS. 


183 


CONVERSATION  VI. 

OF  THE   HYDROSTATIC  BELLOWS. 

F.  I  think  we  have  made  it  sufficiently  clear  that 
the  pressure  of  fluids  of  the  same  kind  is  always  pro- 
portional to  the  area  of  the  base  multiplied  into 
the  perpendicular  height  at  which  the  fluid  stands, 
without  any  regard  to  the  form  of  the  vessel,  or  the 
quantity  of  fluid  contained  in  it. 

E.  1  cannot  help  saying,  that  it  still  appears  very 
mysterious  to  me  that  a  pint  of  water  (in  Fig.  11.) 
should  have  an  equal  pressure  with  the  20  pints  in 
the  next  vessel.  You  will  not  say  that  one  pint 
weighs  as  much  as  the  20. 

F.  Your  objection  is  proper.  The  pressure  of  the 
water  upon  the  bottom  cc,  does  not  in  the  least  alter 
the  weight  of  the  vessel  and  water  considered  as  one 
mass,  for  the  action  and  re-action  which  cause  the 
pressure,  destroy  one  another  with  respect  to  the 
weight  of  the  vessel,  which  is  as  much  sustained  by 
the  action  upwards  as  it  is  pressed  by  the  re-action 
downwards. 

The  pressure  of  water  and  other  fluids  differs  from 
its  gravity  or  weight  in  this  respect :  the  weight  is 
according  to  the  quantity  ;  but  the  pressure  is  accord- 
ing to  the  perpendicular  height. 

C.  Suppose  both  vessels  were  filled  with  any  solid 
substance,  would  the  effect  produced  be  very  dif- 
ferent l 

F.  If  the  water  were  changed  into  ice,  for  instance, 
the  pressure  upon  the  bottom  of  the  smaller  vessel 
would  be  much  less  than  that  upon  the  larger. 

Here  is  another  instrument  to  shew  you  that  a  very 
few  ounces  of  water  will  lift  up  and  sustain  a  large 
weight. 

E,  What  is  the  instrument  called? 

F.  It  is  made  like  common  bellows,  only  without 
valves,  and  writers  have  given  it  the  name  of  the 


184 


HYDROS!  A.TICS. 


hydrostatic  bellows.  This  small  tin 
pipe  eo  communicates  with  the  inside 
of  the  bellows.  At  present  the  upper 
and  lower  board  are  kept  close  to  one 
another  with  the  weight  w.  The  in- 
side of  the  boards  are  not  very  smooth, 


so  that  water  may  insinuate  itself  -p-^ 
between  them:  pour  this  half  pint  of  ^' 
water  into  the  tube. 

C.  It  has  separated  the  boards  and  lifted  up  the 
weight. 

F.  Thus  you  see  that  seven  or  eight  ounces  of 
water  has  raised  and  continues  to  sustain  a  weight  of 
561b.  By  diminishing  the  bore  of  the  pipe,  and  in- 
creasing its  length,  the  same  or  even  a  smaller  quan- 
tity of  water  will  raise  a  much  larger  weight. 

C.  How  do  you  find  the  weight  that  can  be  raised 
by  this  small  quantity  of  water  ? 

F.  Fill  the  bellows  with  water,  the  boards  of  which, 
when  distended,  are  three  inches  asunder :  I  will 
screw  in  the  pipe.  As  there  is  no  pressure  upon  the 
bellows,  the  water  stands  in  the  pipe  at  the  same  level 
with  that  in  the  bellows  at  z. 

Now  place  weights  on  the  upper  board  till  the 
water  ascend  exactly  to  the  top  of  the  pipe  e :  these 
weights  express  the  weight  of  a  pillar  or  column  of 
water,  the  base  of  which  is  equal  to  the  area  of  the 
lower  board  of  the  bellows,  and  the  height  equal  to 
the  distance  of  that  board  from  the  top  of  the  pipe. 

E.  Will  you  make  the  experiment  ? 

F.  Your  brother  shall  first  make  the  calculation. 
C.  But  I  must  look  to  you  for  assistance. 

F,  You  will  require  very  httle  of  my  help.  "Mea- 
sure the  diameter  of  the  bellows,  and  the  perpendi- 
cular height  of  the  pipe  from  the  bottom  board. 

C.  The  bellows  is  circular  and  12  inches  in  diame- 
ter ;  the  height  of  the  pipe  is  36  inches. 

F.  Well  ;  you  have  to  find  the  solid  contents  of  a 
cylinder  of  these  dimensions  :  that  is,  the  area  of  the 
base  multiplied  by  the  height. 


HYDROSTATIC  BELLOWS. 


185 


C.  To  find  the  area  I  multiply  the  square  of  12 
inches,  that  is  144,  by  the  decimals  .7854,  and  the 
product  is  113,  nearly  the  number  of  square  inches 
m  the  area  of  the  bottom  board  of  the  bellows.  And 
113  multiplied  by  36  inches,  the  length  of  the  pipe, 
gives  4068,  the  number  of  cubic  inches  in  such  a  cy- 
linder ;  this  divided  by  1728  (the  number  of  cubic 
inches  in  a  cubic  foot)  leaves  a  quotient  of  2.3  cubic 
feet,  the  solid  contents  of  the  cylinder.  Still  I  have 
not  the  weight  of  the  water. 

F,  The  weight  of  pure  water  is  equal  in  all  parts 
of  the  known  world,  and  a  cubical  foot  of  it  weighs 
1000  ounces. 

C.  Then  such  a  cylinder  of  water  as  we  have  been 
conversing  about  weighs  2300  ounces,  or  144  pounds 
nearly. 

E.  Let  us  now  see  if  the  experiment  answers  to 
Charles's  calculation. 

F,  Put  the  weights  on  carefully,  or  you  will  dash 
the  water  out  at  the  top  of  the  pipe,  and  I  dare  say 
that  you  will  find  the  fact  agrees  with  the  theory. 

C.  If  instead  of  this  pipe,  one  double  the  length 
was  used,  would  the  water  sustain  a  double  weight  ] 

F.  It  would :  and  a  pipe  three  or  four  times  the 
length  w^ould  sustain  three  or  four  times  greater 
weights. 

C,  Are  there  then  no  limits  to  this  kind  of  experi- 
ment except  those  which  arise  from  the  difficulty  of 
acquiring  length  in  the  pipe  ? 

F,  The  bursting  of  the  bellows  would  soon  deter- 
mine the  limit  of  the  experiment.  Dr.  Goldsmith 
says  that  he  once  saw  a  strong  hogshead  split  by  this 
means.  A  strong  small  tube  made  of  tin,  about  20 
feet  long,  was  cemented  into  the  bung-hole,  and  then 
water  was  poured  in  to  fill  the  cask  ;  when  it  was  full 
and  the  water  had  risen  to  within  about  a  foot  of  the 
top  of  the  tube,  the  vessel  burst  with  prodigious  force. 

E.  It  is  very  difficult  to  conceive  how  this  pressure 
acts  with  such  power. 

F.  The  water  at  o  is  pressed  with  a  force  propor- 


iSo  HYDROSTATICS, 
tional  to  the  perpendicular  altitude  e  o  ;  this  pressure 
is  communicated  horizontally  in  the  direction  o 
and  the  pressure  so  communicated  acts  as  you  know 
equally  in  all  directions  :  the  pressure  therefore  down- 
wards upon  the  bottom  of  the  bellows  is  just  the  same 
as  it  would  heii  p  q  n  r  were  a  cylinder  of  water. 

The  experiment  made  on  the  bellows  might,  for 
want  of  such  an  instrument,  be  made  by  means  of  a 
bladder  in  a  box  with  a  moveable  lid. 

E.  TTas  this  property  of  hydrostatics  been  applied 
to  any  practical  purposes  ? 

F.  The  knowledge  of  it  is  of  vast  importance  in  the 
concerns  of  life.  On  this  principle  a  press  of  im- 
mense power  has  been  formed,  (Conversation  XX IT) 
which  we  shall  describe  after  you  are  acquainted  with 
the  nature  and  structure  of  valves.  It  is  used  for  the 
purpose  of  compressing  soft  substances,  such  as  hay, 
cotton,  wool,  and  other  commodities,  which  it  is  neces- 
sary to  transport  on  board  of  ships  ;  also  soft  manu- 
factured goods,  as  silks,  cottons,  woollen  cloths,  &c. 

CONVERSATION  VII. 

OF  THE  PRESSURE  OF  FLUIDS  AGAINST  THE  SIDES  OF 
VESSELS. 

F.  Do  you  recollect,  Charles,  the  law  by  which 
you   calculate  the  accelerated  velocity  of  falling 

bodies  ?* 

C.  Yes ;  the  velocity  increases  in  the  same  pro- 
portion as  the  odd  numbers,  1,  3,  5,  7,  9,  &c.;  that 
is,  if  at  the  end  of  one  second  of  time  it  has  carried 
the  body  through  16  feet,  then  in  the  next  second  the 
body  will  descend  three  times  16,  or  48  feet  ;  in  the 
third  it  will  descend  five  times  16  feet ;  and  in  the 
next  seven  times  16  feet,  and  so  on  continually  in- 
creasing in  the  same  proportion. 

F.  How  many  feet  has  it  fallen  altogether  at  the 
end  of  the  third  second  ? 


*  See  Mechanics,  Conver.  VII.  and  VIII. 


OF  THE  PRESSURE  OF  FLUIDS.  187 

E.  I  recollect  this  very  well  ;  the  whole  space 
through  which  it  will  fall  in  three  seconds  is  nine 
times  16,  or  144  feet;  because  the  rule  is,  that  the 
whole  spaces  described  by  falling  bodies  are  in  pro- 
portion to  the  squares  of  the  times,  and  the  square  of 
three  is  nine,  therefore  if  it  falls  through  16  feet  in  the 
first  second,  it  will  in  three  seconds  fall  through  nine 
times  16,  and  in  five  or  eight  seconds  it  will  descend 
in  the  former  case  through  25  times  16  feet,  and  in 
the  latter  through  64  times  16  feet,  for  25  is  the 
square  of  five,  and  64  is  the  square  of  eight.  The 
example  of  the  arrow  which  you  gave  me  to  work  has 
fixed  the  rule  in  my  mind. 

Well,  then,  what  I  am  going  to  tell  you  will 
tend  to  impress  the  rule  still  stronger  in  your  memory. 

The  pressure  of  fluids  against  the  sides  of  any 
vessel  increases  in  the  same  proportion,  and  is 
governed  by  the  same  laws. 

Suppose  a  6  c  to  be  a  cubical  vessel 
filled  with  water  or  any  other  fluid,  and 
one  of  the  sides  to  be  accurately  divided 
into  any  number  of  equal  parts  by  the 
lines  1,  7  ;  2,  8,  &c. 

Now  if  the  pressure  of  the  v^^ater 
upon  the  part  of  the  vessel  a  I  b  1  he 
equal  to  an  ounce  or  a  pound,  then  the   Fig.  14. 
pressure  upon  the  part  1,  2,  7,  8,  will  be 
equal  to  three  ounces  or  three  pounds  ;  and  the  pres- 
sure upon  the  part  2, 3, 8, 9,  will  be  equal  to  five  ounces 
or  pounds,  and  so  on. 

C.  Then  I  see  the  reason  why  the  other  part  of  the 
rule  holds  true,  viz.  that  the  pressure  against  the 
whole  side  must  vary  as  the  square  of  the  depth  of  the 
vessel. 

F.  Explain  to  us  the  reason. 

C.  The  pressure  upon  the^iVst  part  being  1,  and  that 
upon  the  second  3,  and  that  upon  the  third  5,  then 
the  pressure  upon  the  first  and  second  taken  together 
is  by  addition  4  :  upon  the  first,  second  and  third  it 


188  HYDROSTATICS. 

must  be  9 ;  and  upon  the  first,  second,  third  and  fourtli, 

it  will  be  16;  but  4,  9,  16  are  the  squares  of  2,  3,  4. 

E.  And  the  pressure  upon  the  whole  side  abed 
must  be  36  times  greater  than  that  upon  the  small 
part  a\b  1 , 

C.  And  if  there  are  three  vessels,  for  instance, 
whose  depths  are  as  1,  2,  and  3,  the  pressure  against 
the  side  of  the  second  will  be  four  times  greater  than 
that  against  the  first ;  and  the  pressure  against  the 
side  of  the  third  will  be  nine  times  greater  than  that 
against  the  first. 

F.  That  is  right ;  the  beautiful  simplicity  of  the 
rule,  and  its  being  the  same  by  which  the  accelerating 
velocity  of  falling  bodies  is  governed,  will  make  it 
impossible  that  you  should  hereafter  forget  it. 

The  use  that  I  shall  hereafter  call  you  to  make  of 
the  rule  induces  me  to  put  a  question  to  Emma. 

In  two  canals,  one  five  feet  deep,  and  the  other  1 5, 
what  difference  of  pressure  will  there  be  against  the 
sides  of  these  canals  ? 

E.  The  pressure  against  the  one  will  be  as  the 
square  of  5,  or  25  ;  that  against  the  other  will  be  as 
the  square  of  15,  or  225  ;  now  the  latter  number  di- 
vided by  the  former  gives  9  as  a  quotient,  which  shews 
that  the  pressure  against  the  sides  of  the  deep  canal  is 
nine  times  greater  than  that  against  the  sides  of  the 
shallow  one. 

Can  this  principle  be  proved  by  an  experiment  1 

F.  By  a  very  simple  one  :  this  is  a 
vessel  of  the  same  size  as  the  last ;  the 
bottom  and  side  h  are  wood  mortised 
together ;  the  front  and  opposite  side  are 
glass  carefully  inserted  in  the  wooden  /-^^ 
parts,  and  made  water  tight.    A  thin  rijv^ 
board  c  hangs  by  two  hinges  x  y,  and  is  ^\  v 
held  close  to  the  glass  panes  by  means 
of  the  pulley  and  weight  ly.  The  board  -^^ 
is  covered  with  cloth  and  made  water 
tisfht. 


OF  THE  PRESSURE  OF  FLUIDS  189 
Now  observe  the  exact  weight  which  is  overcome 
when  the  water  is  poured  in  and  rises  to  the  line  1  ; 
then  hang  on  four  times  that  weight,  and  you  will  see 
that  water  may  be  poured  into  the  vessel  till  it  rise  to 
the  line  2,  when  the  side  c  will  give  way  and  let  part 
of  it  out.  ^ 
But  why  does  only  a  part  run  away  ? 
F .  Because  when  a  small  quantity  of  the  water 
has  escaped,  the  weight  20  is  greater  than  the  pres- 
sure of  the  water  against  c,  and  therefore  the  door  c 
will  be  drawn  close  to  the  glass  panes,  and  confine 
the  rest  within  the  vessel. 

You  may  now  hang  on  a  weight  nine  times  greater 
than  the  first,  and  then  the  vessel  will  contain  water 
till  it  rise  up  to  the  mark  3,  when  the  side  will  give 
way  by  the  pressure  and  part  of  the  water  escape. 

C.  You  have  explained  the  manner  of  estimatino- 
the  pressure  of  fluids  against  the  sides  of  a  vessel^; 
by  what  rule  are  we  to  find  the  pressure  upon  the 
bottom  ? 

F.  In  such  vessels  as  those  which  we  have  just 
described,  that  is  where  the  sides  are  perpendicular  to 
the  bottom,  and  the  bottom  parallel  to  the  horizon,  the 
I  pressure  will  he  equal  to  the  weight  of  the  fluid. 
j      E.  If  then  the  vessel  y  s  hold  a  gallon  of  water, 
I  which  weighs  about  eight  pounds,  and  if  the  bottom 
were  made  moveable  like  the  side,  would  a  weight  of 
eight  pounds  keep  the  water  in  the  vessel  ? 

F,  It  would  ;  for  then  there  would  be  an  equilibri- 
um between  the  pressure  of  the  water  and  the  weight. 
And  the  pressure  upon  any  one  side  is  equal  to  half 
the  pressure  upon  the  bottom  ;  that  is,  provided  the 
bottom  and  sides  are  equal  to  one  another. 
C.  Pray,  sir,  explain  how  this  is  made  out. 
F.  The  pressure  upon  the  bottom  is,  as  we  have 
shewn,  equal  to  the  weight  of  the  fluid.    But  we  have 
I  also  shewn  that  the  pressure  on  the  side  grows  less 
,  and  less  continually,  till  at  the  surface  it  is  nothing, 
i  Since  then  the  pressure  upon  the  bottom  is  truly  re- 
;  presented  by  the  area  of  the  base  multiplied  into  the 

1 


190  HYDROSTATICS. 

altitude  of  the  vessel ;  the  pressure  upon  the  side  will 

be  represented  by  the  base  multiplied  into  half  the 

altitude. 

E.  Is  the  pressure  upon  the  four  sides  equal  to 
twice  the  pressure  upon  the  bottom  1 

F.  It  is :  consequently,  the  pressure  of  any  fluid 
upon  the  bottom  and  four  sides  of  a  cubical  vessel  is 
equal  to  three  times  the  weight  of  the  fluid. 

Can  you,  Charles,  tell  me  the  difference  between 
the  weight  and  the  pressure  of  a  conical  vessel  of  water 
standing  on  its  base  1 

C.  The  iveight  of  a  conical  vessel  of  any  fluid  is 
found  by  multiplying  the  area  of  the  base  by  one  third 
part  of  its  perpendicular  height,  and  then  by  the 
specific  gravity  :*  but  the  pressure  is  found  by  multi- 
plying the  base  by  the  whole  perpendicular  height ; 
therefore  the  pressure  upon  the  base  will  be  equal  to 
three  times  the  weight. 

CONVEKSATION  VIII. 

OF  THE  MOTION  OF  FLUIDS. 

F.  We  will  now  consider  the  pressure  of  fluids 
with  regard  to  the  motion  of  them  through  spouting 
pipes,  v/hich  is  subject  to  the  same  law. 

If  the  pipes  at  1  and  4  (Fig.  15.  p.  187.)  be  equal 
in  size  and  length,  the  discharge  of  water  by  the  pipe 
at  4,  will  be  double  that  at  I.  Because  the  velocity 
with  which  water  spouts  out  at  a  hole  in  the  side  or 
bottom  of  a  vessel  is  as  the  square  root  of  the  distance 
of  the  hole  below  the  surface  of  the  water. 

E.  What  do  you  mean  by  the  squnre  root? 

F.  The  square  root  of  any  number  is  that  which 
being  multiplied  into  itself  produces  the  said  number. 
Thus  the  square  root  of  1  is  1  ;  but  of  4  it  is  2  ;  of  9 
it  is  3  ;  and  of  16  it  is  4,  and  so  on. 

C.  Then  if  you  had  a  tall  vessel  of  water  with  a 

*  See  Bonny  castle's  Introduction  to  Mensuration. 


OF  THE  MOTION  OP  FLUIDS.  191 
cock  inserted  within  a  foot  of  the  top,  and  you  v/ished 
to  draw  the  liquid  off  three  times  faster  than  it  could 
be  done  with  that,  what  would  you  do  ? 

F.  I  might  take  another  cock  of  the  same  size,  and 
insert  it  into  the  barrel  at  nine  feet  distance  from  the 
surface,  and  the  thing  required  would  be  done. 

E.  Is  this  the  reason  why  the  water  runs  so  slowly 
out  of  the  cistern  when  it  is  nearly  empty,  in  compari- 
son of  what  it  does  when  the  cistern  is  just  full? 

F.  It  is  :  because  the  more  water  there  is  in  the 
cistern,  the  greater  the  pressure  upon  the  part  where 
the  cock  is  inserted;  and  the  greater  the  pressure, 
the  greater  the  velocity,  and  consequently  the  quan- 
tity of  water  that  is  drawn  off  in  the  same  time. 

In  some  large  barrels  there  are  two  holes  for  cocks, 
the  one  about  the  middle  of  the  cask,  the  other  at  the 
bottom  ;  now  if  when  the  vessel  is  full  you  draw  the 
beer  or  wine  from  both  cocks  at  once,  you  will  find 
that  the  lower  one  gives  out  the  liquor  much  the 
fastest. 

C.  In  v/hat  proportion  ? 

F .  As  the  square  root  of  2  is  greater  than  that  of 
1  ;  that  is,  while  you  had  a  quart  from  the  upper 
cock,  three  pints  nearly  would  run  from  the  lower  one. 

E.  Are  we  then  to  understand,  that  the  pressure 
against  the  side  of  a  vessel  increases  in  proportion 
to  the  square  of  the  depth  ;  but  the  velocity  of  a  spout- 
ing pipe,  which  depends  upon  the  pressure  at  the 
orifice  itself,  increases  only  as  the  square  root  of  the 
depth  1 

F.  That  13  the  proper  distinction. 

C.  Is  not  the  velocity  of  water  running  out  of  a 
vessel  that  empties  itself  continually  decreasing  ? 

F .  Certainly :  because  in  proportion  to  the  quan- 
tity drawn  off,  the  surface  descends,  and  consequently 
the  perpendicular  depths  become  less  and  less. 

The  spaces  described  by  the  descending  surface,  in 
equal  portions  of  time,  are  as  the  odd  numbers  1,  3,  5, 
7,  9,  &c.  taken  backwards. 

If  the  height  of  a  vessel  filled  with  any  fluid 


192 


HYDROSTATICS. 


be  divided  into  25  parts,  and  in  a  given  space  of  time, 
as  a  minute,  the  surface  descend  through  nine  of  those 
parts,  will  it  in  the  next  minute  descend  through  seven 
of  those  parts,  and  the  third  minute  five,  in  the  fourth 
tltree,  and  in  the  fifth  one  1 

F.  This  is  the  law,  and  from  it  have  been  invented 
elepsydrtp^  or  water-clocks. 

C.  How  are  they  constructed? 

F.  Take  a  cylindrical  vessel,  and  having  ascer- 
tained the  time  it  will  require  to  empty  itself,  then 
divide,  by  lines,  the  surface  into  portions,  which  are  to 
one  another  as  the  odd  numbers  1,  3,  5,  7,  &c. 

E.  Suppose  the  vessel  require  six  hours  to  empty 
itself,  how  must  it  be  divided  ? 

F,  It  must  be  first  divided  into  36  equal  parts ;  then, 
beginning  from  the  surface,  take  eleven  of  those  parts 
for  the  first  hour,  nine  for  the  second,  seven  for  the 
third,  five  for  the  fourth,  three  for  the  fifth,  and  one 
for  the  sixth ;  and  you  will  find  that  the  surface  of  the 
water  will  descend  regularly  through  each  of  these 
divisions  in  an  hour. 

I  believe  both  of  you  have  seen  the  locks  that  are 
constructed  on  the  river  Lea  ? 

C.  Yes  :  and  I  have  wondered  why  the  flood-gates 
were  made  of  such  an  enormous  thickness. 

F.  But  after  what  you  have  heard  respecting  the 
pressure  of  fluids,  you  will  see  the  necessity  that  there 
is  for  the  great  strength  employed. 

C.  1  do  :  for  sometimes  the  height  of  the  water  is 
20  or  30  times  greater  on  one  side  of  the  gates  than  it 
is  on  the  other,  therefore  the  pressure  will  be  400  or 
even  900  times  greater  against  one  side  than  it  is 
against  the  other. 

E.  How  are  the  gates  opened  when  such  a  weight 
presses  against  them  ? 

F.  There  is  hardly  any  power  by  which  they  could 
be  moved  when  this  weight  of  water  is  against  them  ; 
therefore  there  are  sluices  by  the  side,  which  being 
drawn  up,  the  water  gets  away  through  them  into  the 
bason,  till  it  becomes  level  on  both  sides  :  then  the 


OP  THE  MOTION  OF  FLUIDS.  193 
o-ates  are  opened  with  the  greatest  ease,  because,  the 
pressure  being  equal  on  both  sides,  a  small  force 
applied  will  be  sufficient  to  overcome  the  friction  of 
the  hinges,  or  other  trifling  obstacles. 

C.  Is  it  this  great  pressure  that  sometimes  beats 
down  the  banks  of  rivers  1 

F.  It  is ;  for  if  the  banks  of  a  river  or  canal  do  not 
mcrease  in' strength  in  the  proportion  of  the  square  of 
the  depth,  they  cannot  stand.  Sometimes  the  water  m 
a  river  will  insinuate  itself  through  the  bank  near  the 
bottom,  and  if  the  weight  of  the  bank  be  not  equal  to 
that  of  the  water  it  will  assuredly  be  torn  up,  perhaps 
with  great  violence. 

I  will  make  the  matter 
clear  by  a  figure.  Suppose 
this  figure  be  a  section  of 
a  river,  and  c  a  crevice  or 


drain  made  by  time  under  '-^^  ^ 
the  bank  ^;  by  what  we  -p-^^ 
have  shewn  before  the  up-  °  . 

ward  pressure  of  the  water  in  that  drain  is  equal  to 
the  downward  pressure  of  the  water  in  the  river  ; 
therefore,  if  that  part  of  the  bank  be  not  as  heavy  as  a 
column  of  water  the  same  height  and  width,  it  must 
be  torn  up  by  the  force  of  the  pressure. 

C.  Is  there  no  method  of  securing  leaks  that  hap- 
pen in  the  embankments  of  rivers  ? 

F.  The  only  method  is  that  called  puddling.  If  n 
be  the  bank  of  a  canal  in  which  a  leak  is  discovered, 
the  water  must  be  first  drawn  off  below  the  leak,  and 
a  trench  18  or  20  inches  wide  dug  length-wise  along 
the  side  of  the  canal,  and  deeper  than  the  bottom  of 
the  canal ;  this  is  filled  by  a  little  at  a  time  with  clay 
or  loam  reduced  into  a  half  fluid  state  by  mixing  it 
with  water :  when  the  first  layer,  which  is  seldom 
above  six  or  eight  inches  deep,  is  nearly  dry,  another 
is  worked  in  the  same  manner  till  the  whole  be  filled. 
By  this  means,  if  the  operation  be  performed  by  skilful 
hands,  and  time  be  allowed  for  all  the  parts  to  dry  and 
cohere,  the  bank  becomes  strong  and  impenetrable. 
K 


19-4 


HYDROSTATICS. 


CONVERSATION  IX. 

OF  THE  MOTION  OF  FLUIDS. 

F.  I  will  now  shew  you  an  experiment  by  which 
you  will  observe  the  uniformity  of  nature's  operations 
in  regard  to  spouting  fluids. 

C.  Do  you  refer  to  any  other  facts  besides  those 
which  relate  to  the  quantity  of  water  issuing  from 
pipes  ? 

F\  Yes,  I  do.  Let  ab 
represent  a  tall  vessel  of  wa- 
ter, which  must  be  always 
kept  full  while  the  experi- 
ments are  making.  From 
the  centre  of  this  vessel,  I 
have  drawn  a  semi-circle, 
the  diameter  of  which  is  the  Fig.  17. 

height  of  the  vessel  a  b.  I 

have  drawn  three  lines,  d  2  from  the  centre  of  the 
vessel ;  c  1,  a  5,  at  equal  distances  from  the  centre, 
the  one  above  and  the  other  below  it :  all  three  are 
drawn  perpendicular  to  the  vessel.  By  taking  out 
the  plug  from  the  centre  you  will  see  that  the  water 
spouts  to  M.  Take  your  compasses,  and  you  will  find 
that  the  distance  n  m  is  exactly  double  the  length  of  d  2. 
I  will  now  stop  this  plug  and  open  the  next  below. 

C.  The  water  reaches  to  k,  which  is  double  the 
length  of  a  5. 

Try  in  the  same  manner  the  pipe  c. 

C,  It  falls  in  the  same  spot  k,  as  it  did  from  the 
lower  one. 

F.  Because  the  lines  c  1  and  a  5  being  equally 
distant  from  the  centre  of  the  semi-circle,  they  are 
equal  to  one  another. 

E.  Then  n  k  is  the  double  of  c  1,  as  well  as  of  a  5. 

F.  It  is  :  the  general  rule  deduced  from  these  ex- 
periments is,  that  the  horizontal  distance  to  which  a 
fluid  will  spout  from  an  horizontal  pipe,  in  any  part 


OF  THE  MOTION  OF  FLUIDS.  195 

of  the  side  of  an  upright  vessel  below  the  surface  of 
the  fluid,  is  equal  to  twice  the  length  of  a  perpen- 
dicular to  the  side  of  the  vessel,  drawn  from  the  moutli 
of  the  pipe  to  a  semi-circle  described  upon  the  altitude 
of  the  vessel. 

Can  you,  Charles,  tell  me  in  what  part  the  pipe 
should  be  placed,  in  order  that  the  fluid  should  spout 
the  farthest  possible. 

C.  In  the  centre  :  for  the  line  d  2  seems  to  be  the 
greatest  of  all  the  lines  that  can  be  drawn  from  the 
vessel  to  the  curved  line. 

F,  Yes,  it  is  demonstrable  by  geometry  that  this  is 
the  case :  and  also,  that  lines  at  equal  distances 
from  the  centre  above  and  below  are  also  equal  to 
each  other. 

E,  Then,  in  all  cases,  if  pipes  are  placed  equally 
distant  from  the  centre  they  will  spout  to  the  same 
point. 

F.  They  will.  Instead  of  horizontal  pipes,'  I  will 
fix  three  others  which  shall  point  obliquely  upwards 
at  different  angles  ;  one  at  22<*  30',  the  second  at  45^, 
and  the  third  at  67^  30',  and  you  will  see  that  when 
I  open  the  cocks,  the  water  will  cut  the  curve  line 
in  those  places  to  which  the  lines  were  drawn. 

C,  That  which  spouts  from  the  centre  is  thrown 
to  the  point  m,  as  it  was  from  the  centre  horizontal 
pipe.  The  two  others  fall  on  the  point  k,  on  which  the 
upper  and  lower  horizontal  pipes  ejected  the  stream. 

E.  I  thought  the  water  from  the  upper  cock  did 
not  reach  so  high  as  the  mark. 

F,  It  did  not.  The  reason  is,  that  it  had  to  pass 
through  a  larger  body  of  air,  and  the  resistance  from 
that  retarded  the  water,  and  prevented  it  from  ascend- 
ing to  the  point  to  which  it  would  have  ascended  if 
the  air  had  been  taken  away. 

While  we  are  on  this  subject,  I  will  just  mention, 
that  as  you  see  the  water  spouts  the  farthest  when  the 
pipe  is  elevated  to  an  angle  of  45*',  so  a  gun,  cannon, 
&c.  will  project  a  bullet  the  farthest,  if  it  be  elevated 
to  an  angle  of  45'^. 


196 


HYDROSTATICS. 


C.  Will  a  cannon  or  mortar  carry  a  ball  equally 
distant  if  it  be  elevated  at  angles  equally  distant 
from  45°,  the  one  above  and  the  other  below  1 

F.  It  will,  in  theory  :  but  owing  to  the  great  resis- 
tance which  very  swift  motions  meet  with  from  the 
air,  there  must  be  allowances  made  for  some  variation 
between  theory  and  practice. 

A  regard  to  this  will  explain  the  reason  why  water 
will  not  rise  so  high  in  a  jet,  as  it  does  in  a  tube, 

E.  I  do  not  know  what  this  means. 

F.  You  have  seen  a  fountain  1 

E.  Yes,  I  have  often  been  amused  with  that  in  the 
Temple. 

F,  All  fountains  are  called  jets,  or  jets  d'eau.  Now 
if  the  water  of  that  in  the  Temple  ascended  in  a  pipe, 
it  would  rise  higher  than  it  does  in  the  open  air.  Turn 
to  Fig.  10.  p.  179 ;  the  water  in  the  small  tube  rises 
to  a  level  with  that  in  the  larger  one  ;  now  if  the 
tube  HG  was  broken  off  at  t,  the  water  would  spout  up 
like  a  fountain,  but  not  so  high  as  it  stands  in  the 
tube,  perhaps  no  higher  than  to  d* 

C,  Is  that  owing  wholly  to  the  resistance  of  the 
air  ? 

F.  It  is  to  be  ascribed  to  the  resistance  which  the 
water  meets  with  from  the  air,  and  to  the  force  of 
gravity,  which  has  a  tendency  to  retard  the  motion  of 
the  stream. 

£.  Why  does  the  fountain  in  the  Temple  some- 
times play  higher  and  sometimes  lower  1 

F.  Near  the  Temple-hall  there  is  a  reservoir  of 
water,  from  which  a  pipe  communicates  with  the  jet 
in  the  fountain :  and  according  as  the  water  in  the 
reservoir  is  higher  or  lower,  the  height  to  which  the 
fountain  plays  is  regulated. 

C,  By  turning  a  cock  near  the  pump  the  fountain 
is  instantly  lowered. 

F,  That  cock  is  likewise  connected  with  the  reser- 
voir, and  therefore  taking  water  from  it  must  have 
the  effect  of  lowering  the  stream  at  the  fountain,  as 
well  as  that  in  the  reservoir. 


OF  THE  MOTION  OF  FLUIDS.  197 

E.  It  soon  recovers  its  force  again. 

F.  Because  there  is  a  constant  supply  of  water  to 
the  reservoir,  which,  however,  does  not  come  in  so 
quick  as  the  cock  lets  it  out,  or  the  fountain  would 
always  play  to  the  same  height. 

From  what  you  have  already  learnt  on  this  subject 
you  will  be  able  to  know  how  London  and  other 
places  are  supplied  with  water. 

C,  London  is,  I  believe,  supplied  from  the  New 
River,  but  I  do  not  know  in  what  manner. 

F.  The  New  River  is  a  stream  of  water  that  comes 
from  Ware  in  Hertfordshire ;  it  runs  into  a  reservoir 
situated  on  the  high  ground  near  Islington.  From 
this  reservoir  pipes  are  laid  into  those  parts  of  town 
that  have  their  water  from  the  New  River,  and 
through  these  pipes  the  water  flows  into  cisterns  be- 
longing to  different  houses. 

E.  Then  the  reservoir  at  Islington  must  be  higher 
than  the  cisterns  in  London, 

F,  Certainly,  because  water  will  not  rise  above  its 
level.  On  this  account  some  of  the  higher  parts  of 
town  have  hitherto  been  supplied  from  ponds  at 
Hampstead  and  Highgate ;  and  others  were,  till 
lately,  supplied  from  the  Thames  by  means  of  the 
water-works  at  London  Bridge, 

C.  Are  pipes  laid  all  the  way  from  Hampstead  to 
town  1 

F,  They  are :  but  these  supply  the  intermediate 
villages  as  well  as  London.  And  Hampstead  stand- 
ing so  high,  the  water  is  carried  up  into  the  first  and 
second  stories  in  some  houses.  Thus  you  see  that 
water  may  be  carried  to  any  distance,  and  houses  on 
different  sides  of  a  deep  valley  may  be  supplied  by 
water  from  the  same  spring-head.  You  must  remem- 
ber that  if  the  valleys  are  very  deep  the  pipes  must  be 
exceedingly  strong  near  the  bottom,  because  the  pres- 
sure increases  in  the  rapid  proportion  of  the  odd 
numbers  1,  3,  5,  7,  &c.  and  therefore,  unless  the 
strength  of  the  wood  or  iron  be  increased  in  the  same 
proportion,  the  pipes  will  be  continually  burstmg. 


IDS  HYDROSTATICS. 

E,  You  told  me  the  other  day,  that  the  large 
mound  of  earth,  for  it  appears  nothing  else,  near  the 
end  of  Tottenham  Court  lload  was  intended  as  a 
reservoir  for  the  New  River. 

-F.  What  appears  to  you,  and  others  who  pass  by 
it,  only  a  mound  of  earth,  is  an  exceedingly  large 
bason,  capable  of  containing  a  great  many  thousand 
hogsheads  of  water. 

C.  How  will  they  get  the  water  into  it  1 

F.  At  Islington  near  the  New  River  Head  is  made 
a  large  reservoir  upon  some  very  high  ground,  into 
which,  by  means  of  a  steam-engine,  they  will  con- 
stantly throw  water  from  the  New  River.  This  reser- 
voir being  higher  than  that  in  Tottenham  Court 
Road,  nothing  more  is  necessary  than  to  lay  pipes 
from  Islington  to  that  place  in  order  to  keep 'it  con- 
stantly full  of  water. 

By  this  contrivance  the  New  River  Company  will 
be  able  to  extend  their  business  to  other  parts  of 
London,  where  their  present  head  of  water  caiHiot 
reach. 

C.  The  weight  of  water  in  this  place  must  be  im- 
mensely great. 

F,  It  is ;  and  therefore  you  observe  what  a  thick- 
ness the  mound  of  earth  against  the  wall  is  towards 
the  bottom,  and  that  it  diminishes  towards  the  top 
as  the  pressure  becomes  less  and  less. 

E,  Would  not  the  consequences  be  very  serious  if 
the  water  were  to  insinuate  itself  through  the  earth  at 
the  bottom  1 

F,  If  such  an  accident  were  to  happen  when  the 
reservoir  was  full  of  water,  it  would  probably  tear  up 
the  works  and  do  incredible  mischief.  To  prevent 
this,  the  vast  bank  of  earth  is  sloped  within,  as  well  as 
without ;  it  will  then  be  covered  with  a  strong  coat- 
ing of  clay ;  after  this  it  will  be  built  up  with\  very 
thick  brick  wall,  which  will  be  carefully  terraced 
over,  so  that  the  whole  mass  will  be  as  firm  and  com- 
pact as  a  glass  bottle. 


OF  SPECIFIC  GRAVITIES. 


199 


CONVERSATION  X. 

OF  THE  SPECinC  GRAVITIES  OF  BODIES. 

E  What  is  the  reason,  papa,  that  some  bodies,  as 
lead  or  iron,  if  thrown  into  the  water,  sink,  while 
others,  as  wood,  will  swim '? 

F.  Those  bodies  that  are  heavier  than  water  will 
sink  in  it,  but  those  that  are  lighter  will  swim, 

E.  I  do  not  quite  comprehend  your  meaning ;  a 
pound  of  wood,  another  of  water,  and  another  of 
lead,  are  all  equally  heavy.  For  Charles  played  me 
a  trick  the  other  day  :  he  suddenly  asked  which  was 
heavier,  a  pound  of  lead,  or  a  pound  of  feathers  :  1 
said  the  lead,  and  you  all  laughed  at  me,  by  which  I 
was  soon  led  to  perceive,  that  a  pound,  or  16  ounces 
of  any  substance  whatever,  must  be  always  equal  to 
the  same  weight.  i     t     i       *  i 

F  You  are  not  the  first  person  that  has  been  taken 
in  by  this  question.  It  is  a  common  trick.  Although 
a  pound  of  lead  and  another  of  water  be  equally 
heavy,  yet  they  are  not  of  equal  magnitudes.  Do 
you  know  how  much  water  goes  to  a  pound  1 

C.  Yes  :  about  a  pint. 

E.  Do  you  think  that  if  I  were  to  fill  the  same  pint 
measure  with  lead,  that  would  weigh  a  pound  only  1 

C,  Oh  no :  that  would  weigh  a  great  deal  more. 
I  do  not  believe  that  the  14  pounds  weight  below 
stairs  is  much  larger  than  a  pint  measure. 

E.  Yes  it  is,  by  about  a  fourth  part ;  the  same 
measure  that  contains  one  pound  of  water  Nould,  how- 
ever, contain  about  11  pounds  of  lead  ;  but  it  would 
contain  14  pounds  of  quicksilver,  which,  you  know,  I 
could  as  readily  pour  into  the  vessel  as  if  it  were  water. 

Here  are  two  cups  of  equal  size  ;  fill  the  one  with 
water,  and  I  will  fill  the  other  with  quicksilver  1 

E.  Why  did  you  not  let  Charles  pour  out  the 
quicksilver  1  r  i 

F.  The  loss  of  water  is  a  matter  of  little  conse- 
quence ;  but  if,  by  chance,  he  had  thrown  down  the 


200 


HYDROSTATICS. 


quicksilver,  the  accident  might  have  occasioned  the 
loss  of  sixpence,  or  a  shilling;  and  economy  is  right 
in  all  the  affairs  of  life.  Take  the  cups  in  your  hand : 
which  is  the  heavier  ? 

C.  The  quicksilver  by  much. 

F.  But  the  two  cups  are  of  equal  size. 

E.  Then  there  must  be  equal  quantities  of  water 
and  quicksilver. 

F,  They  are  equal  in  bulk. 

C.  But  very  unequal  in  weight :  shall  I  try  how 
much  heavier  the  one  is  than  the  other  7 

F,  If  you  please.  In  what  manner  will  you  ascer- 
tain the  matter? 

C.  I  will  carefully  weigh  the  two  cups,  and  then,  di- 
viding the  larger  weight  by  the  smaller,  I  shall  see  how 
many  times  heavier  the  quicksilver  is  than  the  water. 

F.  You  will  not  come  to  the  point  accurately  by 
that  means ,  because  the  weight  of  the  cups  is  proba- 
bly equal,  but  by  this  method  they  ought  to  differ  in 
weight  in  the  same  proportion  as  the  two  substances. 

E.  Then  pour  the  quicksilver  first  into  the  scale 
and  weigh  it ;  afterwards  do  the  same  with  the  water  ; 
and  divide  the  former  by  the  latter :  will  not  that 
give  the  result  1 

F.  Yes,  it  will :  or  you  may  make  the  experiment 
in  this  method. 

Here  is  a  small  phial,  that  weighs,  now  it  is  empty, 
an  ounce ;  fill  it  with  pure  rain  water,  and  the  weight 
of  the  whole  is  two  ounces. 

C.  Then  it  contains  one  ounce  of  water. 

F.  Pour  out  the  water,  and  let  it  be  well  dried 
both  within  and  without :  fill  it  now  very  accurately 
with  quicksilver,  and  weigh  it  again. 

E.  It  weighs  a  little  more  than  15  ounces  :  but  as 
the  bottle  weighs  one  ounce,  the  quicksilver  weighs 
something  more  than  14  ounces. 

F.  What  do  you  infer  from  this,  Charles'? 

C.  That  the  quicksilver  is  more  than  14  times 
heavier  than  water. 

jp.  I  will  now  pour  away  the  quicksilver,  and  fill 


OF  SPECIFIC  GRAVITIES. 


201 


the  phial  with  pure  spirits  of  wine,  or,  as  the  chemists 
call  it,  with  alcohol. 

E.  It  does  not  weigh  two  ounces  now  ;  conse- 
quently the  fluid  does  not  weigh  an  ounce.  The 
alcohol  is,  then,  lighter  than  water. 

F.  By  these  means,  which  you  cannot  fail  of  under- 
standing, we  have  obtained  the  comparative  weights  of 
three  fluids  :  philosophers,  as  I  have  before  told  you, 
call  these  comparative  weights  the  specific  gravities 
of  the  fluids  :  they  have  agreed  also  to  make  pure  rain 
water  the  standard  to  which  they  refer  the  comparative 
weights  of  all  other  bodies,  whether  solid  or  fluid. 

C.  Is  there  any  particular  reason  why  they  prefer 
water  to  every  other  substance  1 

F.  I  told  you  a  few  days  ago  that  rain  water,  if 
very  pure,  is  of  the  same  weight  in  all  parts  of  the 
world  :  and,  what  is  very  remarkable,  a  cubical  foot 
of  water  weighs  exactly  a  thousand  ounces  avoirdu- 
pois :  on  these  accounts  it  is  admirably  adapted  for 
a  standard,  because  you  can  at  once  tell  the  weight 
of  a  cubical  foot  of  any  other  substance,  if  you  know 
its  specific  gravity. 

E.  Then  a  cubical  foot  of  quicksilver  weighs 
14,000  ounces. 

F,  You  are  right ;  and  if  lead  is  11  times  heavier 
than  water,  a  cubical  foot  of  it  will  weigh  11,000 
ounces. 

CONVERSATION  XL 

OP  THE  SPECIFIC  GRAVITIES  OF  BODIES. 

F,  Before  we  enter  upon  the  methods  of  obtaining 
the  specific  gravities  of  different  bodies,  it  will  be 
right  to  premise  a  few  particulars,  which  it  is  neces- 
sary should  be  well  understood. 

^"ou  now  understand,  that  the  specific  gravity  of  dif- 
ferent bodies  depends  upon  the  different  quantities  of 
matter  which  equal  bulks  of  these  bodies  contain. 

C.  As  the  momenta*  of  different  bodies  are  esti- 


*  See  Mc'clianic.;,  Conver.  VI, 

K  2 


202 


HYDROSTATICS. 


mated  by  the  quantities  of  matter  when  the  velocities 
are  the  same  ;  so  the  specific  gravities  of  bodies  are 
estimated  by  the  quantities  of  matter  when  the  bulks 
or  magnitudes  are  the  same.  This,  I  believe,  is  what 
you  mean. 

F,  I  do  ;  if  you  had  a  piece  of  wood,  and  another 
piece  of  lead,  both  exactly  equal  in  size  to  a  copper 
penny-piece,  the  former  would  be  much  lighter,  and 
the  latter  considerably  heavier  than  the  copper. 

C.  And  I  should  say  that  the  specific  gravity  of  the 
wood  is  less  than  that  of  the  copper,  but  of  the  lead 
it  is  greater. 

E.  Is  it  then  the  density  that  constitutes  the  specific 
gravity  1 

F.  Undoubtedly  it  is  ;  and,  as  we  observed  yester- 
day, water  is  made  use  of  as  a  medium  to  discover  the 
different  specific  gravities  of  diflPerent  bodies  ;  and 
also  as  a  standard  to  which  they  may  be  all  referred. 

Here  are  three  pieces  of  different  kinds  of  wood, 
which  I  will  put  into  this  vessel  of  water :  one  sinks 
to  the  bottom  ;  a  second  remains  in  any  position  of 
the  water  in  which  it  is  placed  ;  and  the  third  swims 
on  the  water  with  more  than  half  of  the  substance 
above  its  surface. 

C.  The  first,  then,  is  heavier  than  the  water,  the 
second  is  of  the  same  weight  with  an  equal  bulk  of  the 
fluid,  and  the  third  is  lighter. 

F.  Since  fluids  press  in  all  directions,  a  solid  that 
is  immersed  in  water  sustains  a  pressure  on  all  sides, 
which  is  increased  in  proportion  to  the  height  of  the 
fluid  above  the  solid. 

E.  That  seems  natural,  but  an  experiment  would 
fix  it  better  in  the  mind, 

F,  Tie  a  leathern  bag  to  the  end  of  a  glass 
tube,  and  pour  in  some  quicksilver.  Dip 
the  bag  in  water,  and  the  upward  pressure  of 

the  fluid  will  raise  the  quicksilver  in  the  tube,  '^J 
the  ascent  of  which  will  be  higher  or  lower 
in  proportion  to  the  height  of  the  water 
abov«  the  bag.  Fig.  18 


OF  SPECIFIC  GRAVITIES.  203 

E.  I  now  understand  that,  the  upper  part  of  the 
tube  being  empty,  or,  at  least,  only  filled  with  air, 
the  upward  pressure  of  the  water  against  the  bag 
must  be  greater  than  the  downward  pressure  of  the 
air  :  and  that,  as  the  pressure  increases  according  to 
the  depth,  therefore  the  mercury  must  keep  rising  in 
the  tube. 

What  is  the  reason  that  a  body  heavier  than  water, 
as  a  stone,  sinks  to  the  bottom,  if  the  pressure  up- 
wards is  always  equal  to  that  downwards  1 

F,  This  is  a  very  proper  question.  The  stone  en- 
deavours to  descend  by  the  force  of  gravity  :  but  it 
cannot  descend  without  moving  away  as  much  of  the 
water  as  is  equal  to  the  bulk  of  the  stone  :  therefore  it 
is  resisted,  or  pressed  upwards,  by  a  force  equal  to  the 
weight  of  as  much  water  as  is  equal  in  magnitude  to 
the  bulk  of  the  stone :  but  the  weight  of  the  water  is 
less  than  that  of  the  stone,  consequently  the  force 
pressing  against  it  upwards  is  less  than  its  tendency 
downwards,  and  therefore  it  will  sink  with  the  differ- 
ence of  these  two  forces. 

You  will  now  be  at  no  loss  to  understand  the  rea- 
son why  bodies  lighter  than  water  swim. 

C.  The  water  being  heavier,  the  force  upwards  is 
greater  than  the  natural  gravity  of  the  body,  and  it 
will  be  buoyed  up  by  the  difference  of  the  f6rces. 

F,  Bodies  of  this  kind,  then,  will  sink  in  water  till 
so  much  of  them  is  below  the  surface,  that  a  bulk  of 
water  equal  to  the  bulk  of  the  part  of  the  body  which 
is  below  the  surface,  is  of  a  weight  equal  to  the  weight 
of  the  whole  body. 

E,  Will  you  explain  this  more  particularly  ? 

F,  Suppose  the  body  to  be  a  piece  of  wood,  part 
of  which  will  be  above  and  part  below  the  surface  of 
the  water ;  in  this  state  conceive  the  wood  to  be 
frozen  into  the  water. 

C.  I  understand  you  i  if  the  wood  be  taken  out 
of  the  ice,  a  vacuity  will  be  left,  and  the  quantity 
of  water  that  is  required  to  fill  that  vacuity  will 
weigh  as  much  as  the  whole  substance  of  the  wood. 


204  HYDROSTATICS. 

F.  That  was  what  I  meant  to  have  said. 

There  is  one  case  remaining  : — where  equal  bulks 
of  the  water  and  the  wood  are  of  the  same  weight, 
the  force  with  which  the  wood  endeavours  to  descend, 
and  the  force  that  opposes  it,  being  equal  to  one 
another,  and  acting  in  contrary  directions,  the  body 
will  rest  between  them,  so  as  neither  to  sink  by  its 
own  weight,  nor  to  ascend  by  the  upward  pressure  of 
the  water. 

E.  What  is  the  meaning  of  this  glass 
jar  with  the  images  in  it  ? 

F.  I  placed  it  on  the  table  in  order  to 
illustrate  our  subject  to-day.  You  ob- 
serve, that,  by  pressing  the  bladder  with 
my  hand,  the  three  images  all  sink. 

E.  But  not  at  the  same  moment. 

F.  The  images  are  made  of  glass, 
and  about  the  same  specific  gravity  with 
the  water  surrounding  them,  or  perhaps    Fig.  19. 
rather  less  than  it,  and  consequently  they 

all  float  near  the  surface.  They  are  hollow,  with 
little  holes  in  the  feet.  When  the  air  which  lies  be- 
tween the  bladder  and  the  surface  of  the  water  is 
pressed  by  my  hand,  there  is  a  pressure  on  the  water 
which  is  communicated  through  it,  and  that  part  of  it 
which  lies  contiguous  to  the  feet  of  the  imas^es  will 
be  forced  into  the  bodies,  by  which  their  weight  is  so 
much  increased  as  to  render  them  heavier  than  the 
water,  and  they  descend. 

C,  Why  do  they  not  all  descend  to  the  same 
depths  1 

F,  Because  the  hollow  part  of  the  image  e  is 
larger  than  the  hollow  part  of  d,  and  that  is  larger 
than  that  of  c ;  consequently,  the  same  pressure  will 
force  more  water  into  e  than  into  d,  and  more  into  d 
than  into  c. 

E.  Why  do  they  begin  to  ascend  now  you  have 
taken  your  hand  away  1 

F.  I  said  the  hollow  parts  of  the  images  were 
empty,  which  was  not  quite  correct ;  they  were  full  of 


OF  SPECIFIC  GRAVITIES.  205 
air,  which,  as  it  could  not  escape,  was  compressed 
into  a  smaller  space  when  the  water  was  forced  in  by 
the  pressure  upon  the  bladder.  But  as  soon  as  the 
pressure  is  removed,  the  air  in  the  images  expands, 
drives  out  the  water,  and  they  become  as  light  as  at 
first,  and  will  therefore  rise  to  the  surface. 

CONVERSATION  XII. 

OP  THE  METHODS  OF  FINDING  THE  SPECIFIC  GRAVITY 
OF  BODIES. 

E.  What  are  you  going  to  weigh  with  these  scales  ? 
This  instrument  is  called 

the  hydrostatical  balance  ;  it  dif- 
fers but  little  from  the  balance 
in  common  use.  Some  instru- 
ments of  this  kind  are  more  com- 
plicated, but  the  most  simple 
are  best  adapted  to  my  purpose. 

To  the  beam  two  scale-pans 
are  adjusted,  and  may  be  taken 
off  at  pleasure.    There  is  also  Fig.  20. 

another  pan  a,  of  equal  weight 
with  one  of  the  others,  furnished  with  shorter  strings 
and  a  small  hook,  so  that  any  body  may  be  hung  to  it, 
and  then  immersed  in  the  vessel  of  water  n. 

C.  Is  it  by  means  of  this  instrument  that  you  find 
the  specific  gravity  of  diflferent  bodies  1 

F,  It  is  :  I  will  give  you  the  rule,  and  then  illus- 
trate it  by  experiments.  The  rule  should  be  com- 
mitted to  memory : 

**  Weigh  the  body  first  in  air  ;  that  is,  in  the  com- 
mon way  ;  then  weigh  it  in  water,  observe  how  much 
weight  it  loses  by  being  weighed  in  water,  and  by 
dividing  the  former  weight  by  the  loss  sustained,  the 
result  is  its  specific  gravity  compared  with  that  of 
the  water.'* 

I  will  give  you  an  example.  Here  is  a  guinea: 
it  weighs  in  the  air  129  grains  :  I  suspend  it  by  a 
fine  thread  of  horse-hair  to  the  hook  at  the  bottom  of 


200  HYDROSTATICS. 

the  pan  a,  and  you  see  that,  by  being  immersed  in 

water,  It  weig-hs  only  121-|  grains. 

E.  Then  in  the  water  it  has  lost  of  its  weight  7^ 
grains. 

F.  Divide  129  by  7|,  or,  by  turning  the  i  into 
decimals,  by  7.25. 

C.  But  1  must  add  two  ciphers  to  the  129  grains, 
because  there  must  always  be  as  many  decimals  in 
the  dividend  as  there  are  in  the  divisor.  And  129.00 
divided  by  7.25  gives  for  the  quotient  more  than  17. 

F.  The  gold  is  therefore  more  than  17  times 
heavier  than  water. 

I  do  not  understand  the  reason  of  this. 

F,  In  this  scale  is  a  bason  filled  accurately  to  the 
brim  with  water.  I  will  put  a  piece  of  mahogany  into 
It  very  gently;  any  thing  else  would  answer  the  same 
purpose. 

£.  The  water  runs  over  into  the  scale. 

1^.  So  I  expected  it  would  :  now  every  thing  is  at 
rest,  and  the  bason  is  just  as  full  as  it  was  at  first, 
only  that  the  wood  and  water  together  fill  the  bason, 
whereas  it  was  all  water  before.  I  will  take  away 
the  bason,  and  put  the  mahogany  by  itself  into  the 
other  scale. 

E.  It  balances  the  water  that  run  out  of  the  bason. 
C.  The  mahogany  then  displaced  a  quantity  of 

water  equal  to  itself  in  weight. 

F ,  And  so  did  the  guinea  just  now ;  and  if  yon  had 
taken  the  same  precaution,  you  would  have  found 
that  the  quantity  of  water  equal  in  bulk  to  the  guinea 
weighed  7i  grains,  the  weight  which  it  lost  by  bein? 
weighed  in  the  fluid. 

-E.  Am  I  to  understand  that  what  any  substance 
loses  of  its  weight,  by  being  immersed  in  water,  is 
equal  to  the  weight  of  a  quantity  of  water  of  the 
same  bulk  as  the  substance  itself  ? 

F .  This  is  true,  if  the  body  be  wholly  immersed  in 
water;  and  with  regard  to  all  substances  that  are 
specifically  heavier  than  water,  you  may  take  it  as  an 
axiom,  that     every  body,  when  immersed  in  water, 


OF  SPECIFIC  GRAVITIES.  207 

loses  as  much  of  its  weight,  as  is  equal  to  the  weight 
of  a  bulk  of  water  of  the  same  magnitude." 

I  will  now  place  this  empty  box  on  the  bason  filled 
to  the  edge  with  water,  and,  as  before,  it  drives  over 
a  quantity  of  fluid  equal  in  weight  to  itself.  Put  in 
two  penny-pieces,  and  you  perceive  the  box  sinks 
deeper  into  the  water. 

C.  And  they  drive  more  water  over ;  as  much,  I 
suppose,  as  is  equal  in  weight  to  the  copper  coin. 

B\  Right :  how  long  could  you  go  on  loading  the 
box 

C.  Till  the  weight  of  the  copper  and  box,  taken 
together,  is  something  greater  than  the  weight  of  as 
much  water  as  is  equal  in  bulk  to  the  box, 

F»  You  understand,  then,  the  reason  why  boats, 
barges,  and  other  vessels,  swim  on  water;  and  to 
what  extent  you  may  load  them  with  safety. 

E,  They  will  swim  so  long  as  the  weight  of  the 
vessel  and  its  lading  together,  is  less  than  that  of  a 
quantity  of  water  equal  in  bulk  to  the  vessel. 

F,  Can  you,  Charles,  devise  any  method  to  make 
iron  or  lead  swim,  which  are  so  much  heavier  than 
water  1 

C.  I  think  I  can.  If  the  metal  be  beat  out  very 
thin,  and  the  edges  turned  up,  I  can  easily  conceive 
that  a  box  or  a  boat  of  it  may  be  made  to  swim.  Of 
this  kind  is  the  copper  ball  which  is  contrived  to  turn 
off  the  water  when  the  cistern  is  full. 

E.  I  have  often  wondered  how  that  acts. 

F,  If  upon  reflection  you  could  not  satisfy  your- 
self about  the  mode  of  its  acting,  you  should  have 
asked ;  it  is  better  to  get  information  from  another 
than  to  remain  ignorant. 

The  ball,  though  made  of  copper,  which  is  eight  or 
nine  times  heavier  than  water,  is  beat  out  so  thin,  that 
its  bulk  is  much  lighter  than  an  equal  bulk  of  v/ater. 
By  means  of  a  handle  it  is  fastened  to  the  cock, 
through  which  the  water  flows,  and  as  it  sinks  or  rises, 
it  opens  or  shuts  the  cock. 

If  the  cistern  is  empty,  the  ball  hangs  down,  and 


20S 


HYDROSTATICS. 


the  cock  is  open,  to  admit  the  water  freely.  As  the 
water  rises  in  the  cistern  it  reaches  the  ball,  which, 
being  lighter  than  the  water,  rises  with  it,  and,  by 
rising,  gradually  shuts  the  cock,  and,  if  it  be  properly 
placed,  it  is  contrived  to  shut  the  cock  just  at  the 
moment  the  cistern  is  full. 

In  the  same  way  that  these  balls  are  made  boats 
of  iron  are  now  constructed  at  the  iron-works  in 
Shropshire :  they  will  last  longer  than  wood,  and 
cause  less  friction  in  passing  through  the  water. 

Can  you,  Emma,  find  the  specific  gravity  of  this 
piece  of  silver] 

E,  It  weighs  in  air  318  grains:  I  now  fasten  it  to 
the  hook  with  the  horse-hair,  and  it  weighs  in  water 
288  grains,  which,  taken  from  318,  leave  30,  the 
weight  it  lost  in  water.  By  dividing  318  by  30  the 
quotient  is  about  10|;  consequently,  the  specific  gra- 
vity of  the  silver  is  ten  and  a  half  times  greater  than 
that  of  water. 

F.  What  is  the  specific  gravity  of  this  piece  of 
flint-glass  1    It  weighs  12  pennyweights  in  air. 

C,  And  in  water  it  weighs  only  8,  and  consequently 
loses  4  by  immersion  ;  and  12  divided  by  4  gives  3, 
therefore  the  specific  gravity  of  flint-glass  is  three 
times  greater  than  that  of  water. 

F.  This  is  not  the  case  with  all  flint-glass ;  it 
varies  from  2  to  almost  4. 

Here  is  an  ounce  of  quicksilver  j  let  me  know  its 
specific  gravity  by  the  method  now  proposed. 

E,  Hov/  will  you  manage  that  1  you  cannot  hang 
it  up  on  the  balance. 

F»  But  you  may  suspend  this  glass  bucket  0 
on  the  hook  at  the  bottom  of  a  ;  immerse  it  in  T 
the  water,  and  then  balance  it  exactly  with  4^ 
weights  in  the  opposite  scale. 

I  will  now  put  into  the  bucket  the  ounce, 
or  480  grains  of  quicksilver,  and  see  how 
much  it  loses  in  water.  A 

C.  It  weighs  445  grains,  and  consequently  0 
it  lost  35  gruins  by  immersion ;   and  480 


OF  SPECIFIC  GRAVITIES.  209 
divided  by  35  give  almost  14,  so  that  mercury  is 
almost  14  times  heavier  than  w^ater. 

F.  In  the  same  manner  we  obtam  the  specific 
gravity  of  all  bodies  that  consist  of  small  fragments. 
They  must  be  put  into  the  glass  bucket  and  weighed ; 
and  then,  if  from  the  weight  of  the  bucket  and  body 
in  the  fluid,  you  subtract  the  weight  of  the  bucket, 
there  remains  the  weight  of  the  body  in  the  fluid. 

E.  Why  do  you  make  use  of  horse-hair  to  suspend 
the  substances  with ;  would  not  silk  or  thread  do  as 
well? 

F.  Horse-hair  is  by  much  the  best,  for  it  is  very 
nearly  of  the  same  specific  gravity  as  water ;  and  its 
substance  is  of  such  a  nature  as  not  to  imbibe 
moisture. 


CONVERSATION  XIII. 

OF  THE  METHODS  OF  FINDING  THE  SPECIFIC  GBAVITY 
OF  BODIES. 

C.  I  have  endeavoured  to  find  out  the  specific 
gravity  of  this  piece  of  beech-wood,  but  as  it  will  not 
sink  in  the  water  I  know  not  how  to  do  it. 

F,  It  is  true  that  we  have  hitherto  only  given  rules 
for  the  finding  of  the  specific  gravity  of  bodies  that 
are  heavier  than  water ;  a  little  consideration,  how- 
ever, will  shew  you  how  to  obtain  the  specific  gravity 
of  the  beech.  Canyon  contrive  means  to  sink  the 
beech  in  the  water? 

C.  Yes ;  if  I  join  a  piece  of  lead,  or  other  metal, 
to  the  wood,  it  will  sink. 

-F.  The  beech  weighs  660  grains ;  I  will  annex 
to  It  an  ounce,  or  480  grains  of  tin,  which  in  water 
loses  of  Its  weight  51  grains.  In  air  the  weight  of 
the  wood  and  metal  taken  together  is  1140  grains; 
but  in  water  they  weigh  but  138  grains:  138  taken 
from  1140  leave  1002,  the  difiference  between  the 
weights  in  air  and  in  water. 

C.  I  now  see  the  mode  of  finding  what  I  want. 


210 


HYDROSTATICS. 


The  whole  mass  loses  1002  grains  by  immersion, 
and  the  tin  by  itself  lost  in  water  51  grains  j  therefore, 
the  wood  lost  951  grains  of  its  weight  by  immersion  : 
and  660  grains,  the  weight  of  the  beech  in  air,  di- 
vided by  951,  which  it  may  be  said  to  lose  by  immer- 
sion, leaves  in  decimals  for  a  quotient  .694. 

F.  Then  making  water  the  standard  equal  to  1, 
the  beech  is  .694,  or  nearly  seven-tenths  of  1  :  that 
is,  a  cubic  foot  of  water  is  to  a  cubic  foot  of  beech  as 
1000  to  694,  for  the  one  weighs  1000  ounces,  and 
the  other  694  ounces. 

E.  It  seems  odd  how  a  piece  of  wood  that  weighs 
but  660  grains  in  air,  should  lose  of  its  weight  951 
grains. 

F.  You  must,  in  this  case,  consider  the  weight 
necessary  to  make  it  sink  in  water,  which  must  be 
added  to  the  weight  of  the  wood. 

I  will  now  endeavour  to  make  the  subject  easier  by 
a  different  method. 

This  small  piece  of  elni  I  will  place  be- 
tween the  tongs  that  are  nicely  balanced  on 
the  beam.  The  elm  weighs  36  grains.  To 
detain  it  under  water,  I  must  hang  24  grains 
to  the  end  of  the  lever  on  which  the  tongs 
are  fixed:  then  by  the  Rule  of  Three  I  say, 
as  the  specific  gravity  of  the  elm  is  to  the 
specific  gravity  of  water,  so  is  36,  the  weight  Fig.  22. 
of  the  elm,  to  60,  the  weight  of  the  elm  and 
the  additional  weight  required  to  sink  it  in  water.^ 

E.  You  have  not  obtained  the  specific  gravity  of 
the  elm,  but  a  proportion  only. 

C.  But  three  terms  are  given,  because  the  water  is 
always  considered  as  unity  or  1,  therefore  the  specific 

gravity  of  the  elm  is  J^-^^-l  ==  .  6 

E.  I  do  not  yet  comprehend  the  reason  of  the  pro- 
portion assumed. 

F,  It  is  very  simple.  The  elm  is  lighter  than 
the  water,  but  by  hanging  weights  to  the  side  of 
the  balance,  to  which  it  is  attached,  in  order  to 


OF  SPECIFIC  GRAVITIES.  211 
detain  it  just  under  water,  I  make  the  whole  exactly 
equal  to  the  specific  gravity  of  the  water;  by  this 
means  it  is  evident,  that  the  comparative  gravity  of 
the  elm  is  to  that  of  the  water  as  36  to  60. 

Try  this  piece  of  cork  in  the  same  manner. 

It  weighs  half  an  ounce,  or  240  grains,  in  air  ; 
and  to  detain  the  cork  and  tongs  just  under  water,  I 
am  obliged  to  hang  2  ounces,  or  960  grains,  of  lead 
on  the  lever :  therefore,  the  specific  gravity  of  the 
cork  is  to  that  of  the  water  as  240  is  to  1200 ;  and 
240  divided  by  1200  give  the  decimal  .2. 

Then  the  specific  gravity  of  water  is  5  times 
greater  than  that  of  cork. 

C.  We  have  now  obtained  the  specific  gravities  of 
water,  beech,  elm  and  cork,  which  are  as  1,  .7  nearly, 
.6  and  .2.  ^ 

F,  You  now  understand  the  methods  of  obtaining 
the  specific  gravity  of  all  solids,  whether  lighter  or 
heavier  than  water.  In  making  experiments  upon 
light  and  porous  woods,  the  operations  must  be  per- 
formed as  quickly  as  possible,  to  prevent  the  water 
from  getting  into  the  pores. 

C,  And  you  have  likewise  shewn  us  a  method  of 
getting  the  specific  gravity  of  fluids,  by  weighing  cer- 
tain quantities  of  each. 

F,^  I  have  a  still  better  method :  the  rule  I  will 
give  in  words  :  you  shall  illustrate  it  by  examples  : 

If  the  same  body  be  weighed  in  diflerent  fluids, 
the  specific  gravity  of  the  fluids  will  be  as  the  weights 
lost.'^ 

E,  The  body  made  use  of  must  be  heavier  than  the 
fluids. 

F.  Certainly  •  this  glass  ball  loses  of  its  weight  by 
immersion  in  water  803  grains;  in  milk  it  loses  831 
grains  ;  therefore  the  specific  gravity  of  the  water  is  to 
that  of  milk  as  803  to  831.  Now  a  cubical  foot  of 
water  weighs  1000  ounces  ;  what  will  be  the  weight 
of  the  same  quantity  of  milk  1 

E.  As  803  : 831  :  :  1000  :  12^.^1^  =  1035 
803 

ounces,  nearly. 


HYDROSTATICS. 


F.  Do  you,  Charles,  tell  me  what  is  the  specific 
gravity  of  vsome  spirits  of  wine  which  I  have  here. 

C  The  glass  loses  in  water  803  grains,  in  the 
spirit  of  wine  it  loses  699  grains,  therefore  the  specific 
gravity  of  water  is  to  the  spirit  as  803  is  to  699  ;  and 
to  find  the  weight  of  a  cubical  foot  of  the  spirit,  I  say, 

as  803  :  699  : :  lOOO  :  ^^^^^no^^^  =  ^^0  ounces. 

F.  You  may  now  deduce  the  method  of  comparing 
the  specific  gravities  of  solids  one  with  another  with- 
out making  a  common  standard. 

Here  is  an  ounce  of  lead  and  another  of  tin  :  I  may 
weigh  them  in  any  fluid  whatever  ;  in  water  the  lead 
loses  by  immersion  42  grains,  and  the  tin  63  grains. 

E.  Is  the  specific  gravity  of  the  lead  to  that  of  the 
tm  as  42  to  631 

F.  No :  "  the  specific  gravities  of  bodies  are  to  one 
another  inversely  as  the  losses  of  weight  sustained  :" 
therefore,  the  specific  gravity  of  the  lead  is  to  that  of 
the  tin  as  63  to  42  ;  or  if  a  block  of  lead  weighs  63 
pounds,  the  same  sized  block  of  tin  will  weigh  42 
pounds  only. 

C.  I  think  I  see  the  reason  of  this  :  the  heavier  the 
body,  the  less  it  loses  of  its  weight  by  immersion  ; 
therefore,  of  two  bodies  whose  absolute  weights  are 
tlie  same,  that  is,  each  weighing  an  ounce,  pound, 
&c.,  the  one  which  loses  least  of  its  weight  will  be 
specifically  the  heaviest. 

F,  You  are  right ;  for  the  specific  gravity  of  bodies 
is  as  their  density,  and  their  densities  are  inversely  as 
the  weights  they  lose  by  immersion,  that  is,  the 
body  that  is  most  dense  will  lose  the  least  in  water, 
because  it  displaces  the  least  quantity  of  water  ;  a 
pound  of  copper  occupying  seven  or  eight  times  less 
space  than  a  pound  of  wood,  would  therefore  remove 
seven  or  eight  times  less  water. 


HIERO'S  CROWN. 


213 


CONVERSATION  XIV. 

OF  THE  METHODS  OF  OBTAINING   THE    SPECIFIC  GRA- 
VITY OF  BODIES. 

F.  As  I  have  shewn  you  the  methods  of  finding 
the  specific  gravity  of  almost  all  kinds  of  bodies,  it 
will  be  proper  in  this  and  one  or  two  lessons,  to  shew 
you  the  practical  utility  of  this  part  of  science. 

E.  To  whom  are  we  indebted  for  the  discovery  of 
the  mode  of  performing  these  operations  1 

F.  To  that  most  celebrated  mathematician  of  anti- 
quity, Archimedes. 

C.  Was  he  not  slain  by  a  common  soldier  at  the 
siege  of  Syracuse  ? 

F.  He  was,  to  the  great  grief  of  Marcellus,  the 
Roman  commander,  who  had  ordered  that  his  house 
and  person  should  be  respected  :  but  the  philosopher 
was  too  deeply  engaged  in  solving  some  geometrical 
inquiries  to  think  of  seeking  that  protection  which 
even  the  enemy  intended  for  him. 

E.  Had  he  at  that  time  so  high  a  reputation  as  to 
induce  the  general  of  a  besieging  army  to  give  parti- 
cular orders  for  his  preservation  1 

F,  His  celebrity  was  so  great  among  the  literati  of 
Rome,  that  his  tragical  end  caused  more  real  sorrow 
than  the  capture  of  the  whole  island  of  Sicily  did 

joy- 

We  are  informed  by  history,  that  it  was  by  the 
wisdom  of  Archimedes  that  the  fate  of  Syracuse  was 
Jong  suspended  :  by  his  inventions  multitudes  of  the 
Roman  army  were  killed  and  their  ships  destroyed  : 
and  that  he  made  use  of  burning  glasses,  which,  at 
the  distance  of  some  hundreds  of  yards,  set  the  Ro- 
man vessels  on  fire.* 

*  We  shall  consider  this  subject  at  large  in  our  con- 
versations on  Optics. 


214  HYDROSTATICS. 

C.  I  wonder  then  that  he  was  not  defended  by  his 
fellow-citizens. 

F,  Alas!  my  child,  I  am  sorry  to  say  that  m 
other  countries  as  well  as  Sicily,  there  have  been  in- 
stances in  which  persons  who  have  benefited  then- 
country  as  much  as  Archimedes  have  experienced 
no  more  gratitude  than  he  did. 

It  is  a  fortunate  circumstance  when  the  efforts  of 
philosophy  are  directed  under  able  judgment  to  the 
defence  of  one's  country.  The  Romans  had  no  more 
right  to  plunder  Sicily  than  the  highwayman  has  to 
rifle  your  pockets  or  mine.  In  the  eye  of  reason  and 
justice  all  offensive  war  is  the  most  deliberate  and 
cruel  system  of  robbery  and  murder. 

But  to  return  to  our  subject.  To  Archimedes  the 
world  is  indebted  for  the  discovery  **  That  every  body 
heavier  than  its  bulk  of  water,  loses  so  much  of  its 
weight,  by  being  suspended  in  water,  as  is  equaHo 
the  weight  of  a  quantity  of  water  equal  to  its  bulk.'* 

E.  How  did  he  make  the  discovery  1 

F.  Hiero,  king  of  Syracuse,  had  given  to  a  jew- 
eller a  certain  quantity  of  pure  gold  to  make  a  crown 
for  him.  The  monarch,  when  he  saw  the  crown, 
suspected  the  artist  of  having  kept  back  part  of  the 
gold. 

E.  Why  did  he  not  weigh  itl 

F.  He  did :  and  found  the  weight  right :  but  he 
suspected,  perhaps  from  the  colour  of  the  crown,  that 
some  baser  metal  had  been  mixed  with  the  gold,  and 
therefore  though  he  had  his  weight,  yet  only  a  part  of 
it  was  gold,  the  rest  silver  or  copper.  He  applied  to 
Archimedes  to  investigate  the  fraud. 

C,  Did  he  melt  the  crown,  and  endeavour  to  sepa 
rate  the  metals  ? 

F.  That  would  not  have  answered  Hiero's  inten- 
tions ;  his  object  was  to  detect  the  roguery,  if  any, 
without  destroying  the  workmanship.  While  the 
philosopher  was  intent  upon  the  problem,  he  went, 
according  to  his  custom,  into  tlie  bath,  and  he  ob- 
served tliat  a  quantity  of  water  flowed  over,  which  he 


OF  SPECIFIC  GRAVITY.  215 
thought  must  be  equal  to  the  bulk  of  his  own  body. 
He  instantly  saw  the  solution  of  Hiero's  problem.  In 
raptures  at  tfie  discovery,  he  is  said  to  have  leaped 
from  tiie  water  and  run  naked  through  the  streets  of 
the  city,  shouting  aloud  'Ev^tjxa  !  'EvQijza  I  L  have 
found  it  out !  I  have  found  it  out !  '* 

When  the  excess  of  his  joy  was  abated,  he  got  two 
masses,  one  of  gold  and  the  other  of  silver,  each  equal 
in  weight  to  the  crown ;  and  having  filled  a  vessel 
very  accurately  with  water,  into  which  he  first  dipped 
the  silver  mass,  and  observed  the  quantity  of  water 
that  flowed  over,  he  then  did  the  same  with  the 
gold,  and  found  that  a  less  quantity  of  water  had 
flowed  over  than  before, 

C.  And  was  he,  from  these  trials,  led  to  conclude 
that  the  bulk  of  the  silver  was  greater  than  that  of 
the  gold  1 

F,  He  was ;  and  also,  that  the  bulk  of  water  dis- 
placed was,  in  each  experiment,  equal  to  the  bulk  of 
the  metal.  He  then  made  the  same  trial  with  the 
crown,  and  found,  that  though  of  the  same  weight 
with  the  masses  of  silver  and  gold,  yet  it  displaced 
more  water  than  the  gold,  and  less  than  the  silver. 

E,  Accordingly  he  concluded,  I  imagine,  that  it 
was  neither  pure  gold  nor  pure  silver. 

C.  But  how  could  he  discover  the  proportions  of 
each  metal  1 

F,  I  believe  we  have  no  other  facts  to  carry  us 
farther  into  the  history  of  this  interesting  experiment. 
But  to-morrow  I  will  endeavour  to  explain  and  illus- 
trate the  matter. 


CONVERSATION  XV. 

OF  THE  METHODS  OF  OBTAINING  THE  SPECIFIC  GRA- 
VITY OF  BODIES. 

E.  You  are  to  describe,  to-day,  the  method  of  de- 
tectino-  the  proportion  of  each  metal  if  two  are  mixed 
together  in  one  mass. 


21G 


HYDROSTATICS. 


F.  Suppose  I  take  in  change  a  guinea,  which  I 
suspect  to  be  bad  :  upon  trying  it  I  find  it  weighs  129 
grains,  which  is  the  standard  weight  of  a  guinea.  I 
then  weigh  it  in  water,  and  it  loses  of  its  weight  8^ 
grains,  by  which  I  divide  the  129^  and  the  quotient  is 
15.6,  the  specific  gravity  of  the  guinea.  But  you  know 
the  specific  gravity  of  the  gold,  made  at  the  Mint,  is 
more  than  17,  and  therefore  I  conclude  the  guinea  is 
base  metal,  a  mixture  of  silver,  or  copper,  with  stan- 
dard gold. 

C.  But  how  will  you  get  the  proportions  of  the 

two  metals  ? 

F,  Suppose,  for  example,  that  the  mass  be  a  com- 
pound of  silver  and  gold. — "  Compute  what  the  loss 
of  a  mass  of  standard  gold  would  be  j  and  likewise 
the  loss  which  a  mass  of  silver  equal  in  weight  to  the 
guinea  would  sustain.  Subtract  the  loss  of  the  gold 
from  that  of  the  compound,  the  remainder  is  the  ratio 
or  proportion  (not  the  quantity)  of  the  silver  :  then 
subtract  the  loss  of  the  compound  from  that  of  the 
silver,  the  remainder  is  the  proportion  of  the  gold." 
I  will  propose  you  an  example. 

What  are  the  proportions  of  silver  and  gold  in  a 
guinea  weighing  129  grains,  whose  specific  gravity  is 
found  to  be  only  13.09  ;  supposing  the  loss  of  stand- 
ard gold  7.25,  and  that  of  a  piece  of  silver,  equal  in 
weight  to  a  guinea,  12.45,  and  the  loss  of  the  com- 
pound 9  851 

C.  I  first  subtract  the  loss  of  standard  gold  7.25 
from  the  loss  of  the  compound  9.85,  the  remainder  is 
2.6  :  I  now  take  the  loss  of  the  compound  9.85  from 
that  sustained  by  the  silver  12.45,  and  the  remainder 
is  also  2.6. 

jP.  Then  the  proportions  of  silver  and  gold  are 
equal  to  one  another,  consequently  the  false  guinea 
is  half  standard  gold  and  half  silver. 

Here  is  another  counterfeit  guinea,  which  is  full 
weight,  but  I  know  it  is  composed  of  standard  gold, 
adulterated  with  copper,  and  its  loss  in  water  is, 
as  you  see,  8.64  :  now  tell  me  the  proportions  of  the 


OF  SPECIFIC  GRAVITY. 


217 


two  metals  ;  but  you  should  be  informed,  that  a  piece 
of  copper  of  the  weight  of  a  guinea  would  lose  in 
water  14.65  grains. 

E.  I  deduct  7.25,  the  loss  of  a  guinea  standard 
gold,  from  8.64,  the  remainder  is  1.39  :  I  now  take 
the  loss  of  the  compound  8.64  from  14.65,  the  loss 
sustained  by  a  piece  of  copper  equal  in  weight  to  a 
guinea,  and  the  remainder  is  6.01.  Is  not  the  pro- 
portion of  gold  to  copper  as  1.39  to  6.01  1 

F.  You  are  quite  right.  Now  by  the  rule  of  three 
tell  me  the  quantity  of  each  metal. 

E.  To  find  the  weight  of  the  copper,  I  add  6.01 
and  1.39  together,  which  are  the  proportional  weights 
of  the  two  metals ;  and  say,  as  7.40,  the  sum,  is  to 
1.39,  the  proportional  weight  of  copper,  so  is  the 
weight  of  the  guinea,  129  grains,  to  the  real  weight 
of  copper  contained  in  the  counterfeit  guinea:  but 

1.39  X  jj£---24.1  ;  therefore  there  is  a  little  more 
7.40 

than  24  grains  of  copper  in  the  compound. 

F.  You  have  found  then  that  there  are  24  grains 
of  copper  in  this  counterfeit  guinea.  How  will  you 
find  the  weight  of  the  gold  1 

E.  Very  easily,  for  if  the  composition  be  copper 
and  gold,  and  there  are  found  to  be  24  grains  of  cop- 
per, there  must  be  105  of  gold. 

C.  I  have  a  question  to  propose.  If  by  chance 
you  take  a  bad  guinea  (I  have  heard  you  say  that 
you  never  attempt  to  pass  it  upon  others),  how  should 
you  be  able  to  ascertain  the  value  it  would  fetch 
at  the  goldsmith's  ? 

F.  It  is  certainly  very  wrong  knowingly  to  pass 
bad  money  upon  the  public  :  no  man  has  a  right  to 
commit  an  injury  because  he  has  received  one  ;  if 
therefore  I  have  taken  counterfeit  money,  I  ought  to 
abide  by  the  loss,  rather  than  run  the  risk  of  injuring 
my  neighbour  :  besides,  in  the  course  of  circulation, 
a  bad  guinea  or  a  sovereign,  or  even  coins  of  much 
less  value,  may  fall  into  the  hands  of  a  poor  and  in- 


218 


HYDROSTATICS. 


dustrious  family,  which  they  perhaps  lay  by  to 
answer  the  extraordinary  demands  of  sickness  ;  and 
at  that  period  of  distress  not  being  able  to  say  from 
whom  they  received  the  counterfeit  coin,  they  may 
possibly  be  reduced  to  serious  and  pitiable  difficulties  ; 
and  therefore  it  is  better  for  me  to  put  up  with  the 
loss  than  run  the  hazard  of  injuring  the  poor. 

Now  to  answer  your  question.  A  piece  of  cop- 
per of  equal  weight  with  a  guinea  loses  of  its 
weight  in  water  14.65  grains,  7.4  more  than  is  lost  by 
a  standard  guinea.  The  value  of  a  standard  guinea 
is  252  pence  :  divide  therefore  252  by  7.4,  and  you 
get  34,  the  number  of  pence  that  is  deducted  from 
the  value  of  a  guinea,  for  every  grain  it  loses  more 
than  it  would  lose  if  it  were  sterling  gold. 

E,  In  the  guinea  that  lost  8.64  hov*'  much  must  be 
deducted  from  the  real  value  of  a  guinea  standard 
gold? 

C.  I  can  tell  that:  subtract  7.25  from  8,64  the  re- 
mainder is  1.39,  and  this  multiplied  by  34  pence 
gives  47.26  pence,  or  very  nearly  4  shillings,  conse- 
quently that  guinea  is  worth  only  17  shillings. 

F,  Suppose  the  compound  were  silver  and  gold, 
how  would  you  proceed  in  making  an  estimate  of  its 
value  1 

C.  A  piece  of  silver  of  tlie  weight  of  a  guinea 
would  lose  12.45  grains,  from  which  I  deduct  7.25, 
and  with  the  remainder  5.2  I  divide  the  value  of  a 
guinea,  or  252  pence,  and  the  quotient  is  48.4  pence, 
or  rather  more  than  4  shillings  is  to  be  deducted  from 
the  value  of  a  guinea  adulterated  with  silver,  for 
every  grain  it  loses  by  immersion  more  than  standard 
gold". 

E,  How  is  that  papa  1  silver  is  much  dearer  than 
copper,  and  yet  you  allow  4  shillings  a  grain  when  the 
guinea  is  alloyed  with  silver,  and  but  2s.  lOd.  when 
the  mixture  is  made  with  copper  1 

F,  Because  the  specific  gravity  of  silver  is  much 
nearer  to  that  of  gold  than  that  of  copper ;  conse- 


OF  SPECIFIC  GllAVITIES. 


219 


quently,  if  equal  quantities  of  silver  and  copper  were 
mixed  with  gold,  the  silver  would  cause  a  much  less 
loss  by  immersion  in  water  than  the  copper. 

As  it  seldom  happens  that  the  adulteration  of  metal 
in  guineas  is  made  with  all  copper,  or  with  all  silver, 
but  generally  with  a  mixture  of  both,  three  shillings 
is  upon  the  average  allowed  for  every  grain  that  the 
base  m^etal  loses  by  immersion  in  water  more  than 
sterling  gold. 

E,  There  is  a  silver  cream -jug  in  the  parlour ;  I 
have  heard  mamma  say,  she  did  not  think  it  was  real 
silver ;  how  could  she  fmd  out  whether  she  had  been 
imposed  on  ] 

F,  Go  and  fetch  it.    We  will  nov*'  weigh  it. 

E,  It  weighs  5|  ounces,  but  I  must  weigh  it  in 
water,  and  it  has  lost  in  the  water  10|  dwts  ;  and  di- 
viding 5|  ounces,  or  110  pennyweights,  by  lOJ,  I  get 
for  answer  10.7,  the  specific  gravity  of  the  jug. 

F,  Then  there  is  no  cause  for  complaint,  for  the 
specific  gravity  of  good  wrought  silver  is  seldom  more 
than  this. 


TABLE  OF  SPECIFIC  GRAVITIES. 

Mercury 

.  13.568 

Iron  (bar) 

.  7.788 

Zinc  .... 

.  7.191 

Flint-glass 

.  3.290 

Marble  .      .      •  . 

.  2.700 

.  1.250 

Ash  .... 

.800 

220  HYDROSTATICS. 

Maple  755 

Elm  600 

Fir  550 

Cork  -240 


CONVERSATION  XVI. 

OF  THE  HYDROMETER. 

-F.  Before  I  describe  the  construction  and  uses  of 
the  hydrometer,  I  will  shew  you  an  experiment  or  two 
which  will  afford  you  entertainment,  after  the  dry 
calculations  in  some  of  our  former  conversations. 

C.  The  arithmetical  operations  are  rather  tedious,  to 
be  sure,  but  they  serve  to  bring  to  mind  what  we  have 
already  learnt,  and  at  the  same  time  shew  to  what 
uses  arithmetic  may  be  applied- 

F.  You  know  that  wine  is  specifically 
lighter  than  water,  and  the  lighter  body  will 
always  be  uppermost ;  upon  these  principles, 
I  will  exhibit  two  or  three  experiments  :  I 
have  fdled  the  bulb  b  with  port  wine  to  the 
top  of  the  narrow  stem  .r.  1  now  fill  a  with 
water.  -p-^  23 

E.  The  wine  is  gradually  ascending  like  ®" 
a  fine  red  thread  through  the  water  to  its  surface. 

-F.  And  so  it  will  continue  till  the  water  and  wine 
have  changed  places. 

C.  1  wonder  the  two  liquids  do  not  mix,  as  wine 
and  water  do  in  a  common  drinking  glass. 

jP.  It  is  the  narrowness  of  the  stem  x  which  prevents 
the  admixture :  in  time,  however,  this  would  be 
eflrected,  because  v/ater  and  wine  have  what  the 
chemists  call  an  attraction  for  each  other. 

Here  is  a  small  bottle  b  with  a  neck 
three  inches  long,  and  about  one-sixth  of 
an  inch  wide ;  it  is  full  of  red  wine  ;  I  will 
now  place  it  at  the  bottom  of  a  jar  of 
water,  a  few  inches  deeper  than  the  bottle 
is  high.  The  wine  you  observe  is  ascend-  Fig.  24. 
ing  through  the  water. 

E.  This  is  a  very  pretty  experiment :  the  wine  rises 


OP  THE  HYtpROMETER.  221 

in  a  small  column  to  the  surface  of  the  water,  spread- 
ing itself  over  it  like  a  cloud. 

F.  Now  reverse  the  experiment :  fill  the  bottle  with 
water,  and  plunge  its  neck  quickly  into  a  glass  of 
wine,  with  its  mouth  downwards  j  the  wine  is  taking- 
place  of  the  water. 

C.  Could  you  decanter  a  bottle  of  wine  in  this  way 
without  turning  it  up  ? 

F.  I  could,  if  the  neck  of  the  decanter  were  suf- 
ficiently small.  The  negroes  in  the  West  Indies  are 
said  to  be  well  acquainted  with  this  part  of  hydrosta- 
tics, and  to  plunder  their  masters  of  rum  by  filling 
a  common  bottle  with  water,  and  plunging  the  neck 
of  it  into  the  bung-hole  of  the  hogshead. 

E.  Poor  creatures,  they  ought  to  have  something  to 
console  them  for  the  miseries  they  endure. 

F,  Indeed  the  cruelties  that  are  in  general  exer- 
cised upon  the  slaves  very  much  extenuate  the  crime 
of  pilfering,  of  which  they  are  said  to  be  guilty. 

Upon  the  principle  of  lighter  fluids  keeping  the 
uppermost  parts  of  a  vessel,  several  fluids  may  be. 
placed  upon  one  another  in  the  same  vessel  without 
mixing  :  thus  in  a  long  upright  jar,  three  or  four  inches 
in  diameter,  I  can  place  water  first,  then  port  wine, 
then  oil,  brandy,  oil  of  turpentine,  and  alcohol. 

C.  How  would  you  pour  them  in  one  upon  another 
without  mixing  ] 

-F.  This  will  require  a  little  dexterity  :  when  the 
water  is  in,  I  lay  a  piece  of  very  thin  pasteboard  upon 
its  surface,  and  then  pour  in  the  wine  ;  after  which 
I  take  away  the  pasteboard,  and  proceed  in  the  same 
manner  with  the  rest.  Take  a  common  goblet,  or 
drinking  glass,  pour  water  in,  and  then  lay  a  thin 
piece  of  toasted  bread  upon  the  water,  and  you  may 
pour  your  wine  upon  the  bread,  and  the  two  fluids 
will  remain  for  some  time  separate. 

E.  Is  the  toast  placed  merely  to  receive  the  shock 
of  the  wine  when  poured  in  ? 

F.  That  is  the  reason.  Now  I  will  proceed  to  ex- 
plain the  principle  of  the  hydrometer,  an  instrument 


222  HYDROSTATICS. 

contrived  to  ascertain  with  accuracy  and  expedition 
the  specific  gravities  of  different  fluids. 

A  B  is  a  hollow  cylindrical  tube  of  glass, 
ivory,  copper,  &c.  five  or  six  inches  long, 
annexed  to  a  hollow  sphere  of  copper  d  : 
to  the  bottom  of  this  is  united  a  smaller 
sphere  e,  containing  a  little  quicksilver, 
or  a  few  leaden  shot,  suflficient  to  poise  the 
machine,  and  make  it  sink  vertically  in  the 
fluid. 

C.  What  are  the  marks  on  the  tube  1 

F.  They  are  degrees,  exhibiting  the  mag- 
nitudes of  the  part  below  the  surface,  con- 
sequently the  specific  gravity  of  the  fluid 
in  which  it  descends.  If  the  hydrometer,  when 
placed  in  water,  sinks  to  the  figure  10,  and  in  spirits 
of  wine  to  11.1,  then  the  specific  gravity  of  the  water 
is  to  that  of  the  spirit  as  11.1  to  10.  For  if  the 
same  body  float  upon  different  fluids,  the  specific 
gravity  of  these  fluids  will  be  to  each  other  inversely 
as  the  parts  of  the  body  immersed. 

E.  By  inversely,  do  you  mean  that  the  fluid  in 
which  the  hydrometer  sinks  the  deepest  is  of  the  least 
specific  gravity? 

jP.  Yes  1  do  :  here  is  a  piece  of  dry  oak,  which  if  I 
put  into  spirits  of  wine  is  entirely  immersed  ;  in  water 
the  greatest  part  of  it  sinks  below  the  surface  ;  but  in 
mercury  it  scarcely  sinks  at  all.  Hence  it  is  evident 
that  the  hydrometer  will  sink  deepest  in  the  fluid  that 
is  of  the  least  specific  gravity. 

To  render  this  instrument  of  more  service,  a  small 
stem  is  fixed  at  the  end  of  the  tube,  upon  which  weights 
like  that  at  g  may  be  placed.  Suppose  then  the 
weight  of  the  instrument  is  10  dwts.  and  by  being 
placed  in  any  kind  of  spirit  it  sinks  to  a  certain  point 
L,  it  will  require  an  additional  weight,  suppose 
1.6  dwt.  to  sink  it  to  the  same  depth  in  water:  in 
this  case  the  specific  gravity  of  the  water  to  the  spirit 
will  be  as  11.6  to  10.  By  the  addition  of  different 
weights  the  specific  gravity  of  any  kind  of  liquor  is 


OF  THE  HYDROMETER.  223 

easily  found.  The  point  l  should  be  so  placed  as  to 
mark  the  exact  depth  to  which  the  instrument  will 
sink  in  the  liquor  that  has  the  least  specific  gravity. 

C.  But  you  always  make  the  specific  gravity  of 
water  1,  for  the  sake  of  a  standard. 

F,  Kight :  and  to  find  the  specific  gravity  of  the 
spirit  compared  with  water  at  1,  I  say  as  11  6  :  1  :  : 
10:  .862  nearly,  so  that  I  should  put  the  specific 
gravity  of  this  spirit  down  at  .862  in  a  table  wherp 
water  was  marked  1  :  and  as  a  cubic  foot  of  water 
weighs  1000  ounces,  a  cubic  foot  of  this  spirit  would 
weigh  862  ounces,  which  is  generally  the  standard  of 
pure  rectified  spirit, 

E.  Is  this  what  is  usually  called  spirits  of  wine 

F,  JVo  ;  it  is  the  alcohol  of  the  chemists,  one  pint 
of  which  added  to  a  pint  of  water  make  a  quart  nearly 
of  common  spirits  of  wine. 

C.  You  said  .862  was  generally  the  specific  gravity 
of  alcohol :  what  causes  the  difference  at  other  times  ? 

F.  It  is  not  always  manufactured  of  equal 
strength  ;  and  the  same  fluids  vary  in  respect  to  their 
specific  gravity  by  the  difterent  degrees  of  heat  and 
cold  in  the  atmosphere.  The  cold  of  winter  con- 
denses the  fluid  and  increases  the  specific  gravity ; 
the  heat  of  summer  causes  an  expansion  of  the  fluid, 
and  a  diminution  of  its  specific  gravity. 

F.  You  said  just  now  that  a  pint  of  water  added  to 
a  pint  of  alcohol  made  nearly  a  quart  of  spirits  of 
wine;  surely  two  pints  make  a/u//  quart? 

F,  Irideed  they  will  not.  A  pint  of  water  added 
to  a  pint  of  water  will  make  a  quart :  and  a  pint  of 
spirit  added  to  a  pint  of  spirit  will  make  a  quart :  but 
mix  a  pint  of  spirit  with  a  pint  of  water,  and  there  is  a 
certain  chemical  union  or  penetration  between  the 
particles  of  the  two  fluids,  so  that  they  will  not  make 
a  quart.  This  subject  we  will  resume  in  our  Chemical 
Conversations.* 


*  See  Dialogues  on  Chemistry. 


22-1 


HYDROSTATICS. 


CONVERSATION  XVII. 

■      OF  THE  HYDROMETER,  AND  SWIMMING. 

C.  To  what  purposes  is  the  hydrometer  applied  ? 

F.  It  is  used  in  breweries  and  distilleries  to  ascer- 
tain the  strength  of  their  different  liquors :  and  by 
this  instrument  the  excise  officers  gauge  the  spirits, 
and  thereby  determine  the  duties  to  be  paid  to  the 
revenue. 

I  think,  from  the  time  we  have  spent  in  considering 
the  specific  gravity  of  different  bodies,  you  will  be  at 
no  loss  to  account  for  a  variety  of  circumstances  that 
will  present  themselves  to  your  attention  in  the  com- 
jnon  concerns  of  life.  Can  you,  Emma,  explain  the 
theory  of  floating  vessels  ? 

E,  All  bodies  whatever  that  float  on  the  surface  of 
the  water  displace  as  much  fluid  as  is  equal  in  weight 
to  the  weight  of  the  bodies  ;  therefore,  in  order  that  a 
vessel  may  keep  above  water,  it  is  only  necessary  to 
take  care  that  the  vessel  and  its  cargo,  passengers, 
&c.  should  be  of  less  weight  than  the  weight  of  a 
quantity  of  water  equal  in  bulk  to  that  part  of  the 
vessel  which  it  will  be  safe  to  immerge  in  the  water. 

F.  Salt  water,  that  is,  the  water  in  the  sea,  is 
specifically  heavier  than  fresh  or  river  water. 

C.  Then  the  vessel  will  not  sink  so  deep  at  sea  as 
It  does  in  the  Thames. 

F.  That  is  true;  if  a  ship  is  laden  at  Sunderland, 
or  any  other  sea-port,  with  as  much  coals  or  corn  as 
it  can  carry,  it  will  come  very  safely  till  it  reach  the 
fresh  water  in  the  Thames,  and  there  it  will  infallibly 
go  to  the  bottom  unless  some  of  the  cargo  be  taken 
out. 

E.  How  much  heavier  is  sea  water  than  the  fresh  ? 

F.  About  one-thirtieth  part,  which  would  be  a 
guide  to  the  master  of  a  vessel,  who  was  bent  upon 
freighting  it  as  deeply  as  possible. 

C.  In  bathing,  I  have  often  tried  to  swim,  but  have 


OF  SWIMMING. 


225 


not  yet  been  able  to  accomplish  the  task  :  is  my  body 
specifically  heavier  than  the  water  ? 

F.  I  hope  you  will  learn  to  swim,  and  well  too  ;  it 
may  be  the  means  of  saving  your  own  life,  and  rescu- 
ing others  who  are  in  danger  of  drowning. 

J5y  some  very  accurate  experiments  made  by  Mr. 
Robertson,  a  late  librarian  of  the  Royal  Society,  upon 
ten  different  persons,  the  mean  specific  gravity  of  the 
human  body  was  found  to  be  about  one-ninth  less 
than  that  of  common  river  water. 

C.  Why  then  do  I  sink  to  the  bottom?  I  ought  to 
swim  like  wood  on  the  surface. 

F,  Though  you  are  specifically  lighter  than  water, 
yet  it  will  require  some  skill  to  throw  yourself  into 
such  a  position  as  to  cause  you  to  float  like  wood. 

C.  What  is  that  position  *! 

F.  Dr.  Franklin  recommends  a  person  to  throw 
himself  in  a  slanting  position  on  his  back,  but  his 
whole  body,  except  the  face,  should  be  kept  under 
water. 

Unskilful  persons  in  the  act  of  attempting  this  are 
apt  to  plunge  about  and  struggle  :  by  this  means  they 
take  water  in  at  their  mouths  and  nostrils,  which  of 
itself  would  soon  render  them  as  heavy,  or  heavier, 
than  the  water.  Moreover  the  coldness  of  the  stream 
tends  to  contract  the  body  ;  perhaps  fear  has  the  same 
tendency;  all  these  things  put  together  will  easily 
account  fpr  a  person  sinking  in  the  water. 

E.  But  if  a  dog  or  cat  be  thrown  into  the  pond 
they  seem  as  terrified  as  I  should  be  in  a  like  situa- 
tion, yet  they  never  fail  in  making  their  way  out  by 
swimming. 

F.  Of  all  land  animals  man  is  probably  the  most 
helpless  in  this  element.  The  brute  creation  swim 
naturally:  the  human  race  must  acquire  the  art  by 
practice.  In  other  animals  the  trunk  of  the  body  is 
large,  and  their  extremities  small  :  in  man  it  is  the 
reverse,  the  arms  and  legs  are  small  in  proportion  to 
the  bulk  of  the  body,  but  the  specific  gravity  of  the 
extremities  is  greater  than  that  of  the  trunk,  conse- 

L  2 


226 


HYDROSTATICS. 


quently  it  will  be  more  difficult  for  man  to  keep  above 
water  than  for  four-footed  animals  :  besides,  the  act  of 
swimming  seems  more  natural  to  them  than  to  us,  as 
it  corresponds  more  nearly  to  their  mode  of  walking 
and  running  than  to  ours. 

C.  I  will  try  the  next  time  I  bathe  to  throw  myself 
on  my  back  according  to  Dr.  Franklin's  directions. 

F.  Do  not  forget  to  make  your  experiments  in 
water  that  is  not  so  deep  as  you  are  high  by  at  least 
a  foot,  unless  you  have  an  experienced  person  with 
you ;  because  an  unsuccessful  experiment  in  this 
element,  where  it  is  but  a  little  out  of  your  depth,  may 
be  the  last  you  will  m.ake.  And  neither  your  sister 
nor  I  can  spare  you  yet. 

C.  I  once  jumped  into  a  part  of  the  New  Kiver, 
which  I  thought  did  not  appear  deeper  than  you  say, 
and  I  found  it  was  over  my  head ;  but  there  were 
several  persons  there  who  soon  put  me  in  shallower 
water. 

F.  It  is  not  so  generally  known  as  it  ought  to  be, 
that  the  depth  of  a  clear  stream  of  water  is  always 
one-fourth  part  greater  than  it  appears  to  be.* 

C.  If  the  river  appear  to  be  only  three  feet  deep, 
may  I  reckon  upon  its  being  full  four  feet? 

F.  Yes  ;  you  must  estimate  it  in  this  manner. 
Remember  also,  that  if  a  person  sink  slowly  in  water 
ever  so  deep,  a  small  effort  will  bring  him  up  again, 
and  if  he  be  then  able  to  throw  himself  on  his  back, 
keeping  only  his  face  above  water,  all  will  be  well : 
but  if  instead  of  this  he  is  alarmed,  and  by  struggling 
throw  himself  so  high  above  the  water  that  his  body  does 
not  displace  so  much  of  it  as  is  equal  to  his  weight,  he 
will  sink  with  an  accelerated  motion  :  a  still  stronger 
effort,  which  the  sense  of  danger  will  inspire,  may 
bring  him  up  again,  but  in  two  or  three  efforts  of  this 
kind  his  strength  fails,  and  he  sinks  to  rise  no  more 
alive. 

*  The  reason  of  this  deception  will  be  explained  in 
our  conversations  on  Optics  :  Conversation  IV. 


OF  THE  SYPHON. 


227 


E.  Is  It  the  upward  pressure  which  brings  up  a 
person  that  is  at  a  considerable  depth  in  the  v/ater  ? 

F.  It  is  :  this  upward  pressure  balances  the  weight 
of  water  which  he  sustains,  or  he  would  be  crushed  to 
pieces  by  it.  ■  ,      •  i. 

Cork  an  empty  bottle  ever  so  well,  and  with  weights 
plunge  it  down  a  hundred  yards  into  the  sea,  and 
the  p'ressure  of  the  water  wiU  force  the  cork  into  the 
bottle. 


.4 
ll 


CONVERSATION  XVIII. 

OF  THE  SYPHON. 

F,  This  bended  tube  is  called  a  sy-  /^v 
phon,  and  it  is  used  to  draw  off  water,  dp 
wine,  or  other  fluids,  from  vessels  which     ^  '  - 
it  would  be  inconvenient  to  move  from 
the  place  in  which  they  stand.  ^ 

C.  I  do  not  see  how  it  can  draw  liquor  jf 
out  of  any  vessel :— why  is  one  leg  longer    Yicr,  26. 
than  the  other  1 

F.  I  will  first  shew  you  how  the  operation  is  per- 
formed, and  then  endeavour  to  explain  the  principle. 
I  fill  the  tube  edc  with  water,  and  then  placing  a 
finger  on  e,  and  another  on  c,  I  invert  the  tube,  and 
immerse  the  shorter  leg  into  a  jar  of  water ;  and 
having  taken  my  fingers  away  you  see  the  water  runs 
over  in  a  stream. 

E.  Will  it  continue  to  flow  over1 

•  F.  It  will  till  the  water  in  the  vessel  comes  as  low 
as  E,  the  edge  of  the  syphon. 

C.  Is  this  accounted  for  by  pressure? 

F,  To  the  pressure  or  weight  of  the  atmosphere  we 
are  indebted  for  the  action  of  the  syphon,  pumps,  &c. 
At  present  you  must  take  it  for  granted  that  the  air 
which  we  breathe,  thdugh  invisible,  has  weight,  and 
that  the  pressure  occasioned  by  it  is  equal  to  about  14 


228 


HYDROSTATieS. 


or  15  pounds  upon  every  square  inch.*  The  surface 
of  this  table  is  equal  to  about  six  square  feet,  or  864 
square  inches,  and  the  pressure  of  the  atmosphere 
upon  it  is  equal  to  at  least  12,000  pounds. 

E.  How  does  the  pressure  of  the  air  cause  the 
water  to  run  through  the  syphon  1 

F,  The  principle  of  the  syphon  is  this :  the  two 
legs  are  of  unequal  length,  consequently  the  weight 
of  water  in  the  longer  leg  is  greater  than  that  in  the 
shorter,  and  therefore  will,  by  its  own  gravity,  run 
out  at  c,  leaving  a  vacuum  from  d  to  e,  did  not  the 
pressure  of  the  atmosphere  on  the  surface  of  the 
water  in  the  jar  force  it  up  the  leg  de,  and  thus  con- 
tinually supply  the  place  of  the  water  in  dc. 

C.  But  since  the  pressure  of  fluids  acts  in  all  direc- 
tions, is  not  the  upward  pressure  of  the  atmosphere 
against  c,  the  mouth  of  the  tube,  equal  to  the  down- 
ward pressure  on  the  surface  of  the  water  ? 

jP,  The  pressure  of  the  atmosphere  may  be  con- 
sidered as  equal  in  both  cases.  But  these  equal 
pressures  are  counteracted  by  the  pressures  of  the  two 
unequal  columns  of  water,  de  and  dc.  And  since 
the  atmospheric  pressure  is  more  than  sufficient  to 
balance  both  these  columns  of  fluid,  that  which  acts 
with  the  lesser  force,  that  is,  the  column  de,  vAW  be 
more  pressed  against  dc,  than  dc  is  against  de  at  the 
vertex  d  ;  consequently  the  column  de  will  yield  to 
the  greater  pressure,  and  flow  off  through  the  orifice  c. 

E.  Would  the  same  thing  happen  if  the  outer  leg 
DC  were  shorter  than  the  other  ] 

F.  If  DC  were  broken  oflT  at  b,  even  with  the  sur- 
face of  the  water,  no  water  would  run  over  :  or  if  it 
were  broken  olf  any  where  lower  than  b,  it  would 
only  run  away  till  the  surface  of  the  fluid  descended 

*  If  any  of  my  young  readers  are  unwilling-  to  admit 
this  assertion  without  proof,  they  mnst  be  referred  to  the 
4th,  5th,  and  Cth  Conversations  on  Pneumatics  of  these 
Dialogues,  for  a  complete  demonstration  of  the  fact. 


OF  THE  SYPHON,  229 
to  a  level  vi^ith  the  length  of  the  outer  tube,  because 
then  the  column  de  will  be  no  more  pressed  against 
DC,  than  DC  is  against  de,  and  consequently  the  s}'- 
phon  will  empty  itself,  the  water  in  the  outer  leg  will 
run  out  at  the  lower  orifice,  and  that  in  the  inner  will 
fall  back  into  the  jar. 

C.  In  decanting  a  bottle  of  wine,  are  you  obliged 
first  to  fill  the  syphon  with  liquor,  and  then  invert  it? 

-F.  No  :  a  small  pipe  is  fixed  to  the  outer  leg  of 
the  syphon,  by  which  the  air  is  drawn  out  of  it  by  the 
mouth,  and  the  short  leg  being  immersed  in  the  wine, 
the  fluid  will  follow  the  air,  and  run  out  till  the  bottle 
is  empty. 

The  syphon  is  sometimes  disguised  for  the  sake  of 
amusing  young  people.  Tantalus's  cup 
is  of  this  kind.  The  longer  leg  of  the  sy- 
phon passes  through  and  is  cemented  into 
the  bottom  of  the  cup  :  if  water  be  poured 
into  the  cup,  so  as  not  to  stand  so  high  as 
the  bend  of  the  tube,  the  water  will  remain 
as  in  any  common  vessel ;  but  if  it  be 
raised  over  the  bended  part  of  the  syphon,  Fig.  27. 
it  will  run  over,  and  continue  to  run  til] 
the  vessel  is  emptied.  Sometimes  a  little  figure  of  a 
man,  representing  Tantalus,  conceals  the  syphon,  so 
that  Tarftalus,  as  in  the  fable,  stands  up  to  his  chin  in 
water,  but  is  never  able  to  quench  his  thirst,  for 
just  as  it  comes  to  a  level  with  his  chin,  it  runs  out 
through  the  concealed  syphon. 

This  is  another  kind  of  Tantalus's 
cup,  but  the  syphon  is  concealed  in 
the  handle,  and  when  the  water  in  '2" 
the  cup,  which  communicates  with 
the  shorter  leg  at  c,  is  raised  above 
the  bend  of  the  handle,  it  runs  out 
through  the  longer  leg  at  p,  and  so 
continues  till  the  cup  is  empty. 
This  cup  is  often  made  to  deceive 
the  unwary,  who,  by  taking  it  up  to  drink,  cause  the 
water,  which  was,  while  at  rest,  below  the  bend  of 


230  HYDROSTATICS. 

tlie  syphon,  to  run  over,  and  then  there  is  no  means 

of  stopping  the  stream  till  the  vessel  is  empty. 

C.  I  have  frequently  seen  at  the  doors  of  public 
houses  large  hogsheads  of  spirits  in  carts  or  waggons, 
and  persons  drawing  off  the  contents  by  means  of  an 
instrument  like  a  syphon. 

F.  That  is  called  a  distiller's 
crane  or  syphon,  b  represents 
one  of  these  barrels  with  the 
crane  at  work  from  the  bun^- 
hole  n.  The  longer  leg  mr  is 
about  three  feet  long,  with  a 
stop-cock  near  the  middle, 
which  must  be  shut,  and  then 
the  shorter  leg  is  immersed  in 
the  Hquor. 

E.  Is  the  air  in  the  short  leg  forced  into  the  other 
by  the  upward  pressure  of  the  fluid. 

F.  It  is  ;  and  the  cock  being  shut  it  cannot  escape, 
but  will  be  very  much  condensed.  If  then  the  cock 
be  suddenly  opened,  the  condensed  air  will  rush  out, 
and  the  .pressure  of  the  air  on  the  liquor  in  the  vessel 
will  force  it  over  the  bend  of  the  syphon,  and  cause 
it  to  flow  off  in  a  stream,  as  the  figure  represents.  If, 
however,  the  barrel  be  not  full,  or  nearly  so,  then  it 
is  necessary  to  draw  the  air  out  of  the  syphon  by 
means  of  a  small  tube,  ah,  fixed  to  it. 

By  the  principle  of  the  syphon  we  are  enabled  to 
explain  the  nature  of  intermitting  springs. 

E.  What  are  these,  papa  ? 

F.  They  are  springs,  or 
rather  streams,  that  flow  pe- 
riodically. A  figure  will  give 
a  clearer  idea  of  the  subject 
than  many  words  without. 
GFc  represent  a  cavity  in 
the  bowels  of  a  hill,  or 
mountain,  from  the  bottom 
of  which,  c,  proceeds  the 
irregular  cavity  ced,  forni- 


Fior. 


OF  THE  DIVER'S  BELL.  231 
ing  a  sort  of  natural  syphon.  Now,  as  this  cavity 
fills,  by  means  of  rain  or  snow  draining  through  the 
pores  of  the  ground,  the  water  will  gradually  rise  in 
the  leg  CE,  till  it  has  attained  the  horizontal  level  hh, 
when  it  will  begin  to  flow  through  the  leg  ed,  and 
continue  to  increase  in  the  quantity  discharged  as  the 
water  rises  higher,  till  a  full  stream  is  sent  forth,  and 
then,  by  the  principle  of  the  syphon,  it  must  continue 
to  flow  till  the  water  sink  to  the  level  ii,  when  the  air 
will  rush  into  the  syphon  and  stop  its  motion. 

C.  And  being  once  brought  so  low,  it  cannot  run 
over  again  till  the  cavity  is  full  of  water,  or,  at 
least,  up  to  the  level  hh,  which,  as  it  is  only  supplied 
by  the  draining  of  the  water  through  the  ground, 
must  take  a  considerable  length  of  time.  Is  that  the 
reason  why  they  are  called  intermitting  springs  1 

F,  It  is ;  Mr.  Clare,  in  his  treatise  "Oh  the  Mo- 
tion of.  Fluids,"  illustrates  the  subject  by  referring  to 
a  pond  at  Gravesend,  out  of  which  the  water  ebbs  all 
the  time  the  tide  is  coming  into  the  adjacent  river, 
and  runs  in  while  the  tide  is  going  out.  Another  in- 
stance mentioned  by  the  same  author  is  a  spring  in 
Derbyshire,  called  the  Wedding-well,  which,  at  cer- 
tain seasons,  issues  forth  a  strong  stream,  with  a  sing- 
ing noise,  for  about  three  minutes,  and  then  stops 
again.  At  Lambourn,  in  Berkshire,  there  is  a  brook 
which  in  summer  carries  down  a  stream  of  water  suf- 
ficient to  turn  a  mill ;  but  during  the  winter  there  is 
scarcely  any  current  at  all. 


CONVERSATION  XIX. 

OF  THE  diver's  BELL. 

Take  this  ale-glass,  and  thrust  it  with  the  mouth 
downwards  into  a  glass  jar  of  water,  and  you  will  per- 
ceive that  but  very  little  water  will  enter  into  it. 

C.  The  water  does  not  rise  in  it  more  than  about  a 
quarter  of  an  inch  :  if  I  properly  understand  the  sub- 
ject, the  air  which  filled  the  glass  before  it  was  put  in 


232 


HYDROSTATICS. 


water  is  now  compressed  into  the  smaller  space  ;  and 
it  is  this  body  of  air  that  prevents  more  water  getting 
into  the  glass. 

F.  That  is  the  reason :  for  if  you  tilt  the  glass  a 
little  on  one  side,  a  part  of  the  air  will  escape  in  the 
form  of  a  bubble,  and  then  the  water  will  rise  higher 
in  the  glass. 

Upon  this  simple  principle  machines  have  been  in- 
vented, by  which  people  have  been  able  to  walk 
about  at  the  bottom  of  the  sea,  with  as  much  safety 
as  upon  the  surface  of  the  earth.  The  original  ma- 
chine of  this  kind  was  much  improved  by  Dr.  Halley, 
more  than  a  century  ago  ;  it  was  called  the  Diver's 


C.  Was  it  made  in  the  shape  of  a  bell? 

F,  It  was  ;  and  as  great  strength  was  required 
to  resist  the  pressure  of  the  water,  he  caused  it  to  be 
made  of  copper  ;  this  is  a  representation  of  it.  The 


diameter  of  the  bottom  was  five  feet,  that  of  the  top 
three  feet,  and  it  was  eight  feet  high  :  to  make  the 
vessel  sink  vertically  in  water,  the  bottom  was  loaded 
with  a  quantity  of  leaden  balls. 

E.  It  was  as  large  as  a  good  sized  closet;  but  how 
did  he  contrive  to  get  light  ? 

F.  Light  was  let  into  the  bell  by  means  of  strong 
spherical  glasses  fixed  in  the  top  of  the  machine. 


Bell. 


Fig.  31. 


OF  THE  DIVER'S  BELL.  2'3S 
C.  How  were  the  divers  supplied  with  air  ? 
F.  Barrels,  filled  with  fresh  air,  were  made  suffi- 
ciently heavy,  and  sent  down,  such  as  that  repre- 
sented by  c  ;  from  which  a  leathern  pipe  communi- 
cated with  the  inside  of  the  bell,  and  a  stop-cock  at  the 
upper  part  of  the  bell,  let  out  the  foul  air. 

E.  The  little  men  seem  to  sit  very  contentedly 
under  the  bell,  yet  I  do  not  think  I  should  like  a 
journey  with  them. 

F.  Perhaps  not ;  but  the  principal  mconvenience 
which  divers  experience  arises  from  the  condensation 
of  the  air  in  the  bell,  which  though  in  the  ale-glass 
was  very  trifling,  yet  at  considerable  depths  in  the 
sea  is  very  great,  and  produces  a  disagreeable  pres- 
sure upon  all  parts  of  the  body,  but  more  particu- 
larly in  their  ears,  as  if  quills  were  thrust  into  them. 
This  sensation  does  not  last  long,  for  the  air  pressing 
through  the  pores  of  the  skin,  soon  becomes  as  dense 
within  their  bodies  as  without,  when  the  sense  of  pres- 
sure ceases. 

E.  They  might  stop  their  ears  with  cotton. 

F.  One  of  them  once  thought  himself  as  cun- 
ning as  you,  and  for  the  want  of  cotton  he  chewed 
some  paper,  and  stuflfed  it  into  his  ears  :  as  the  bell 
descended,  the  paper  was  forcibly  pressed  into  the 
cavities,  and  it  was  with  great  difficulty  and  some 
danger  that  it  was  extracted  by  a  surgeon. 

C.  Are  divers  able  to  remain  long  under  water  ? 

F.  Yes  :  when  all  things  are  properly  arranged, 
if  business  require  it  they  will  stay  several  hours 
without  the  smallest  difficulty. 

£.  But  how  do  they  get  up  again  ? 

F.  They  are  generally  let  down  from  on  board 
ship,  and  taking  a  rope  with  them,  to  which  is  fixed 
a  bell  in  the  vessel,  they  have  only  to  pull  the  string, 
and  the  people  in  the  ship  draw  them  up. 

C.  What  does  the  figure  E  represent? 

F.  A  man  detached  from  the  bell,  with  a  kind  of 
inverted  basket  made  of  lead,  in  which  is  fixed  another 
flexible  leathern  pipe,  to  give  him  fresh  air  from  the 


234  HYDROSTATICS. 

bell  as  often  as  he  may  find  it  necessary.  By  this 
method  a  man  may  walk  to  the  distance  of  80  or  100 
yards  from  the  machine. 

E.  It  is  to  be  hoped  his  comrades  will  not  forget 
to  supply  him  with  air. 

F.  If  his  head  is  a  little  above  that  part  of  the  bell 
to  which  the  pipe  communicates,  he  can  by  means  of 
a  stop-cock  assist  himself  as  often  as  he  requires  a 
new  supply  :  and  that  man  is  always  best  helped  who 
can  help  himself. 

C.  I  dare  say  that  is  a  right  principle  ;  in  the  pre- 
sent case,  I  am  sure,  it  would  be  exceedingly  wrong 
to  depend  on  another  for  that  which  might  be  done 
by  one's  self. 

CONVERSATION  XX. 

OF  THE  diver's  BELL. 

F.  You  see  how,  by  this  contrivance,  the  parts  of 
wrecked  vessels  and  their  cargoes  are  saved  from  the 
devouring  ocean  ;  and  by  what  means  people  are 
enabled  to  pursue  the  business  of  pearl  and  coral 
fishing. 

E,  Have  there  been  no  accidents  attending  this 
business  ] 

F.  There  are  very  few  professions,  hovrever  simple, 
the  exercise  of  which,  either  through  carelessness  or 
inattention,  is  not  attended  with  danger.  The  diving 
bell  proved  fatal  to  Mr.  Spalding,  and  an  assistant, 
who  went  down  to  view  the  wreck  of  the  Imperial 
East  Indiaman,  near  Ireland.  They  had  been  down 
twice,  but  on  descending  the  third  time  they  remained 
about  an  nour  under  water,  and  had  two  barrels  of 
air  sent  down  to  them,  but  on  signals  from  below  not 
being  again  repeated,  after  a  certain  time,  they  were 
drawn  up  by  their  assistants,  and  both  found  dead  in 
tlie  bell.  This  accident  happened  by  the  twisting  of 
some  ropes,  which  prevented  the  unfortunate  suffer- 
ers from  announcing  their  wants  to  their  companions 


Fig.  32. 


OF  THE  DIVER'S  BELL.  235 
in  the  ship.— Mr.  Day  also  perished  at  Plymouth  in 
a  diving  bell  of  his  own  construction,  in  which  he  was 
to  have  continued,  for  a  wager,  twelve  hours,  one 
hundred  feet  deep  in  water. 

C.  Did  these  accidents  put  an  end  to  the  experi- 
ments ? 

F.  No ;  but  they  have  led  to  im- 
provements in  the  structure  and  use  of 
the  machine.  Mr.  Smeaton  very  suc- 
cessfully made  use  of  a  square  cast- 
iron  chest,  the  weight  of  which,  50 
cwt.,  was  heavy  enough  to  sink  itself. 
It  was  4J  feet  in  height,  the  same 
number  of  feet  in  length,  and  three 
feet  wide,,  and  of  course  afforded  suf- 
ficient  room  for  two  men  to  work  under  it  at  a  time. 

£.  What  are  those  round  things  at  the  top  ] 

F.  They  are  four  strong  pieces  of  glass  to  admit 
the  light.  The  great  advantage  which  this  had  above 
Dr  Haliey's  bell  was,  that  the  divers  were  supplied 
with  a  constant  influx  of  air,  without  any  attention  of 
their  ovvn,  by  means  of  ^a  forcing  air-pump,  worked 
in  a  boat  upon  the  water's  surface. 

G.  That  is  not  represented  in  the  plate. 
F.  Look  to  the  next  figure,  which 

is  a  diving  machine  of  a  different 
construction,  invented  by  the  very 
ino-enious  and  truly  respectable  lec- 
turer, Mr.  Adam  Walker,*  by  whose 
politeness  1  am  enabled  to  copy  the 
figure. 

This  machine  is  of  the  shape  of  a 
conical  tub,  but  little  more  than  one- 
third  as  large  as  Mr.  Smeaton*s. 
'  The  balls  at  the  bottom  are  lead, 
sufficiently  heavy  to  make  it  sink  of 
itself;  a  bended  metal  tube,  6c,  is 


*  See  Walker's  System  of  Natural  Philosophy, 
2  vols.  4to. 


236 


HYDROSTATICS. 


attached  to  the  outside  of  the  machine,  with  a  stop- 
cock, and  a  flexible  leathern  tube  to  the  other  end 
c ;  this  tube  is  connected  with  a  forcing  air-pump  d, 
which  abundantly  supplies  the  diver  with  fresh  air. 
jE.  Can  he  move  about  with  the  machine  1 
F.  IMost  readily;  for  the  pressure  of  the  water 
being  equal  on  all  sides,  he  meets  with  very  little  re- 
sistance ;  and  the  ropes  and  leathern  tube  being 
fiexible,  he  can,  with  the  machine  over  his  head, 
walk  about  several  yards,  in  a  perpendicular  posture ; 
and  thus  having  a  more  ready  access  to  pieces  of  the 
wreck  than  in  a  cumbrous  bell,  he  can  easily  fasten 
ropes  to  them,  an#perform  any  sort  of  business  nearly 
as  well  as  on  dry  land.  Mr.  Walker  says,  that  the 
greatest  part  of  the  wreck  saved  fxom  the  rich  ship 
-Belgioso  was  taken  up  by  means  of  this  .bell.  The 
following  anecdote,  given  by  this  gentleman,  will  en- 
tertain my  young  readers. 

"  As  the" diver  had  plenty  of  air  to  spare,  he  thought 
a  candle  might  be  supported  in  the  bell,  and  he  could 
descend  by  night.  He  made  the  experiment,  and 
presently  found  himself  surrounded  by  fish,  some  very 
large,  and  many  such  as  he  had  never  seen  before. 
They  sported  about  the  bell,  and  smelt  at  his  legs  as 
they  hung  in  the  water  :  this  rather  alarmed  him,  for 
he  was  not  sure  but  some  of  the  larger  might  taie  a 
fancy  to  him  ;  he  therefore  rang  his  bell  to  be  taken 
up,  and  the  fish  accompanied  him  with  much  good 
nature  to  the  surface." 


OF  PUMPS. 


237 


CONVERSATION  XXI 

OF  PUMPS. 

F.  Here  is  a  glass  model  of  a  com 
mon  pump  which  acts  by  the  pressure 
of  the  atmosphere  on  the  surface  of  the 
water  in  which  it  is  placed. 

E.  Is  this  like  the  pump  below  stairs? 

F.  The  principle  is  exactly  the  same :         j  i-^ 
a  represents  a  rmg  of  wood,  or  metal,  f^^J^^ 
with  pliable  leather  fastened  round  it  to  — 
fit  the  cylinder  A.    Over  the  whole  is  a 

valve  of  metal  covered  with  leather,    Fig.  34. 
of  which  a  part  serves  as  a  hinge  for  the 
valve  to  open  and  shut  by. 
C,  What  is  a  valve,  sir  ? 

F.  It  may  be  described  as  a  kind  of  lid  or  trap- 
door, th^t  opens  one  way  into  a  tube,  but  which  the 
more  forcibly  it  is  pressed  the  other  way,  the  closer 
the  aperture  is  shut :  so  that  it  either  admits  the  en- 
trance of  a  fluid  into  the  tube,  and  prevents  its  return  ; 
or  permits  it  to  escape,  and  prevents  its  re-entrance. 

Attend  now  to  the  figure  :  the  handle  and  rod  r 
end  in  a  fork,  which  passes  through  the  piston,  and 
is  screwed  fast  to  it  on  the  under  side.  Below  this, 
and  over  a  tube  of  a  smaller  bore,  as  s,  is  another 
valve  V  opening  upward,  which  admits  the  water  to 
flow  up,  but  not  to  run  down. 

E.  That  valve  is  open  now,  by  which  we  see  the 
size  of  the  lower  tube,  but  I  do  not  perceive  the  upper 
valve. 

F.  It  is  supposed  to  be  shut,  and  in  this  situation 
the  piston  a  is  drawn  up,  and  being  air-tight,  the 
column  of  air  on  its  top  is  removed,  and  consequently 
leaves  a  vacuum  in  the  part  of  the  cylinder  between 
the  piston  and  the  lower  valve. 

C.  I  now  see  the  reason  of  lifting  up  the  handle  to 


238  HYDROSTATICS, 
pamp:  because  the  piston  then  goes  down  to  the 
lower  valve,  and  by  its  ascent  afterwards  the  vacuum 
is  produced. 

F.  And  the  closer  the  piston  is  to  the  lower  valve 
the  more  perfect  will  be  the  vacuum. 

You  know  there  is  a  pressure  of  the  air  on  all 
bodies  on  or  near  the  surface  of  the  earth,  equal  to 
about  14  or  15  pounds  on  every  square  inch  :  this 
pressure  upon  the  water  in  the  well,  into  which  the 
lower  end  of  the  pump  is  fixed,  forces  the  water  into 
the  tube  z  above  its  level  as  high  as  /. 

C.  What  becomes  of  the  air  that  was  in  that  part 
of  the  tube  1 

F.  You  shall  see  the  operation  :  I  will  put  the 
model  into  a  dish  of  water  which  now  stands  at  a  level 
in  the  tube  z,  with  the  water  in  the.dish.  I  draw  up 
the  piston  a,  which  causes  a  vacuum  in  the  cylinder  a. 

E.  But  the  valve  v  opens,  and  now  the  water  has 
risen  as  high  as  I. 

F.  Because,  when  the  air  was  taken  out  of  the 
cylinder  a,  there  was  no  pressure  upon  the  valve  v  to 
balance  that  beneath  it,  consequently  the  air  in  the 
tube  z  opens  its  valve  v,  and  part  of  it  rushed  into  a. 
But  as  soon  as  part  of  the  air  had  left  the  tube  s,  the 
pressure  of  the  atmosphere  upon  the  water  in  the 
dish  was  greater  than  that  of  the  air  in  the  tube,  and 
therefore  by  the  excess  of  pressure  the  water  is  driven 
into  it  as  high  as  /.  _ 

C.  The  valve  v  is  again  shut. 

F,  That  is  because  the  air  is  diffused  equally  be- 
tween the  level  of  the  water  at  /  and  the  piston  a,  and 
therefore  the  pressures  over  and  under  the  valve  are 
equal.  And  the  reason  that  the  water  rises  no  higher 
than  /  is,  that  the  air  in  that  space  is  not  only  equally 
diffused,  but  is  of  the  same  density  as  the  air  without. 
Push  down  the  piston  a  again. 

E.  I  saw  the  valve  in  the  piston  open. 

E.  For  the  air  between  the  piston  and  valve  v 
could  not  escape  by  any  other  means  than  by  lifting 
up  the  valve  in  a,    I  will  draw  up  the  piston. 


OF  PUMPS.  239 

C.  The  water  has  risen  now  above  the  valve  v  as 
high  as  ?«. 

.F.  I  dare  say  you  can  tell  the  cause  of  this  ? 

C.  Is  it  this  :  by  lifting  up  the  piston,  the  air  that 
was  between  /  and  the  valve  v  rushed  into  a,  and  the 
external  pressure  of  the  atmosphere  forced  the  water 
after  it  ? 

F.  And  now  that  portion  of  air  remains  between 
the  surface  of  the  water  m,  and  the  piston.  The  next 
time  the  piston  is  forced  down  all  the  air  must  escape, 
the  water  will  get  above  the  valve  in  the  piston,  and 
in  raising  it  up  again  it  will  be  thrown  out  of  the 
spout. 

E.  Will  the  act  of  throwing  that  open  the  lower 
valve  again,  and  bring  in  a  fresh  supply  ? 

F,  Yes :  every  time  the  piston  is  elevated,  the 
ower  valve  rises,  and  the  upper  valve  falls ;  but 
every  time  the  piston  is  depressed,  the  lower  valve 
falls,  and  the  upper  one  rises. 

E.  This  method  of  raising  water  is  so  simple  and 
easy,  that  I  wonder  people  should  take  the  trouble  of 
drawing  water  up  from  deep  wells,  when  it  might  be 
obtained  so  much  easier  by  a  pump. 

F.  I  was  going  to  tell  you,  my  child,  that  the 
action  of  pumps,  so  beautiful  and  simple  as  it  is,  is 
very  limited  in  its  operation.  If  the  water  in  the 
well  be  more  than  32  or  33  feet  from  the  valve  v,  you 
may  pump  for  ever,  but  without  any  effect. 

C.  That  seems  strange  ;  but  why  33  feet  in  par- 
ticular ? 

F.  I  have  already  told  you  that  it  is  the  weight  of 
the  atmosphere  which  forces  the  water  into  the  vacu- 
um of  the  pump  :  now  if  this  weight  were  unlimited, 
the  action  of  the  pump  would,  be  so  likewise  ;  but  the 
weight  of  the  atmosphere  is  only  about  14  or  15 
pounds  on  every  square  inch  ;  and  a  column  of  water, 
of  about  33  feet  in  height,  and  whose  surface  is  one 
square  inch,  weighs  also  14  or  15  pounds. 
I  C.  Then  the  weight  of  the  atmosphere .  would 
balance  or  keep  in  equilibrio  only  a  column  of  water 


240  HYDROSTATICS, 
of  33  feet  high,  and  consequently  could  not  support 
a  greater  column  of  water,  much  less  have  power  to 
raise  it  up. 

F.  The  operation  is  entirely  effected  by  the  atmos- 
phere pressing  on  the  surface  of  the  water,  by  which 
it  is  forced  into  the  place  which  the  air  previously 
occupied. 

E,  A  pump,  then,  would  be  of  no  use  in  the  deep 
wells  which  we  saw  near  the  coast  in  Kent. 

F,  None  at  all :  the  piston  of  a  pump  should  never 
be  set  to  work  more  than  28  feet  above  the  water, 
because  at  some  periods  the  pressure  of  the  atmos- 
phere is  so  much  less  than  at  others,  that  a  column  of 
water  something  more  than  28  feet,  will  be  equal  to 
the  weight  of  the  air. 

CONVERSATION  XXII. 

OF  TPIE  FORCING-PUMP,  FIRE-ENGINE,  ROPE-PUMP, 
CHAIN-PUMP,  AND  WATER-PRESS. 

C.  Why  is  this  called  the  forcing- 
pump  ? 

F.  Because  it  not  only  raises  the 
water  into  the  barrel  like  the  common 
pump,  but  afterwards  forces  it  up  into 
the  reservoir  k  k. 

E.  How  is  that  operation  performed, 
papa  ] 

F.  The  pipe  and  barrel  are  the 
same  as  in  the  other  pump,  but  the 
piston  has  no  valve ;  it  is  solid  and 
heavy,  and  made  air-tight,  so  that  no  water  can  get 
above  it. 

C.  Does  the  water  come  up  through  the  valve  a, 
as  it  did  in  the  last? 

F,  By  raising  up  the  piston,  or,  as  it  is  generally 
called,  the  plunger  g,  a  vacuum  is  made  in  the  lower 
part  of  the  barrel,  into  which,  by  the  pressure  of  the 
air,  the.  water  rushes  from  the  well,  as  you  shall  see. 

£.  And  the  valve  is  shut  down. 


OF  THE  FORCING  PUMP. 


241 


F.  The  water  not  being  able  to  go  back  again,  and 
being  a  fluid  that  is  nearly  incompressible,  when  the 
plunger  is  forced  down  it  escapes  along  the  pipe  m 
and  through  the  valve  b  into  the  vessel  k. 

C.  Though  the  water  stands  no  higher  than  h,  yet 
it  flows  through  the  pipe  f  to  some  height. 

F.  The  pipe  f  i  is  fixed  into  the  top  of  the  vessel, 
and  is  made  air-tight,  so  that  no  air  can  escape  out  of 
it  after  the  water  is  higher  than  i,  the  edge  of  the 
pipe. 

E.  Then  the  whole  quantity  of  air  which  occupied 
the  space  f  6  is  compressed  into  the  smaller  space 
h  F. 

F.  You  are  right,  and  therefore  the  extra  pressure 
on  the  water  in  the  vessel  forces  it  through  the  pipe, 
as  you  see. 

C.  And  the  greater  the  condensation,  that  is,  the 
more  water  you  force  into  the  vessel  k,  the  higher  the 
stream  will  mount. 

jP.  Certainly  :  for  the  forcing  pump  difl^ers  from 
the  last  in  this  respect,  that  there  is  no  limit  to  the 
altitude  to  which  water  may  be  thrown,  since  the  air 
may  be  condensed  to  almost  any  degree. 

The  water-works  at  London-bridge,  alluded  to 
p.  197,  exhibited  a  most  curious  engine,  constructed 
on  the  principle  of  the  forcing-pump :  the  wheel- 
work  was  so  contrived  as  to  move  either  way  as  the 
water  ran?  by  these  works  140,000  hogsheads  of 
water  were  raised  every  day. 

E.  Is  there  any  rule  to  calculate  the  height  to 
which  an  engine  will  throw  water  ? 

E.  If  the  air's  condensation  be  double  that  of  the 
atmosphere,  its  pressure  will  raise  water  33  feet ;  if 
the  condensation  be  increased  three-fold,  the  water 
will  reach  66  feet ;  and  so  on,  allowing  the  addition 
of  33  feet  in  height  foi  every  increase  of  one  to  the 
number  that  expressed  the  air's  condensation. 

C.  Are  fire-engines  made  in  this  manner  ? 

E.  They  are  all  constructed  on  the  same  principle, 
M 


242 


HYDROSTATICS. 


but  there  are  two  barrels  by  which  the  water  is 
alternately  driven  into  the  air-vessels  :  by  this  means 
the  condensation  is  much  greater;  the  water  rushes 
out  in  a  continued  stream,  and  with  such  velocity, 
that  a  raging  fire  is  rather  dashed  out  than  extin- 
guished by  it.  Garden-engines  are  also  constructed 
on  a  principle  similar  to  that  we  have  been  describing. 

This  figure  is  the  representation  of  a 
method  of  raising  water  from  wells  of  con-  a 
siderable  depth.  z&^^bz 

E.  Is  it  a  more  convenient  method 

than  the  wheel  and  axis  ?  f  i 

F.  The  wheel  and  axis  are  adapted  ^Jt^ 
merely  to  draw  up  water  by  buckets  ;  li~7j 
whereas  the  rope-pump  is  intended  to  L^^l 
throw  water  into  a  reservoir  to  almost  any 

height.  It  consists  of  three  hair-ropes  Fig.  35. 
passing  over  the  pulleys  a  and  b,  which 
have  three  grooves  in  each.  The  lower  pulley  b  is 
immersed  in  the  water,  in  which  it  is  kept  suspended 
by  a  weight  ^r.  The  pulleys  are  turned  round  with 
great  velocity  by  multiplying  wheels,  and  the  cords 
in  their  ascent  carry  up  a  considerable  quantity  of 
water,  which  they  discharge  into  the  box  or  reservoir 
Zy  from  whence  by  pipes  it  may  be  conveyed  else- 
where. The  ropes  must  not  be  more  than  about  an 
mch  apart. 

E.  What  is  the  reason  of  that,  papa  1 

F.  Because,  in  that  case,  a  sort  of  column  of  water 
will  ascend  between  the  ropes,  to  which  it  adheres  by 
the  pressure  of  the  atmosphere. 

C.  Ought  not  tliis  column,  in  its  ascent,  to  fall 
back  by  its  own  gravity  ? 

F.  Yes  ;  and  so  it  would  did  not  the  great  velocity 
of  the  ropes  occasion  a  considerable  rarefaction  of  the 
air  near  them,  consequently  the  adjacent  parts  of  the 
atmosphere  pressing  towards  the  vacuity,  tend  to  sup- 
port the  water. 

E.  Can  any  considerable  quantity  of  water  be 
raised  in  this  way  1 


OF  THE  WATER-PRESS.  213 
F,  At  Windsor  a  pump  of  this  kind  will  raise,  by  the 
efforts  of  one  man,  about  9  gallons  of  water  in  a  minute 
from  a  well  95  feet  deep.  In  the  beginning  of  motion, 
the  column  of  water  adhering  to  the  rope  is  always  less 
than  when  it  has  been  worked  for  some  time,  and 
the  quantity  continues  to  increase  till  the  surrounding 
air  partakes  of  its  motion.  There  is  also  another  of 
these  pumps  at  the  same  place,  which  raises  water 
from  the  well  in  the  round  tower  178  feet  in  depth. 
C.  What  is  a  chain-pump  1 

P.  It  consists  of  two  square  barrels,  through  which 
a  chain  passes,  having  several  flat  pistons,  or  valves, 
fixed  to  it,  at  certain  distances.  The  chain  passes 
round  wheel-work,  which  is  fixed  at  one  end  of  the 
machine.  A  row  of  the  pistons,  which  are  free  of  the 
sides  of  the  barrel,  is  always  rising  when  the  pump  is 
at  work  ;  and  as  it  is  usually  worked  with  great  velo- 
city, they  bring  up  a  full  bore  of  water  in  the  pump. 

C.  What  are  the  chief  purposes  for  which  the  chain- 
pump  is  used  1 

F.  It  has  been  used  in  the  Navy,  to  prevent  a 
repetition  of  the  fatal  accidents  which  have  sometimes 
occurred  on  shipboard  by  the  choking  of  pumps  witli 
valves.  It  is  adapted  to  raise  water  in  all  situations 
where  it  is  mixed  with  sand,  or  other  substances, 
which  destroy  common  pumps,  as  in  alum-works, 
mines,  quarries,  &c.  It  is  now  much  improved — is 
simple  and  durable — and  may  be  made  of  metal  or 
wood. 

C.  You  told  us  some  time  ago  that  when  we  had 
seen  the  nature  and  understood  the  construction  of 
valves,  you  would  explain  the  action  of  the  water-press. 

F,  This  is  a  good  time  for  the  purpose,  and  with  it 
I  shall  conclude  our  Hydrostatical  Couversations. 

a  is  a  strong  cast-iron  cylinder, 
ground  very  accurately  within,  that 
the  piston  e  may  fit  exceedingly 
close  and  well.  I  need  scarcely 
tell  you  that  the  little  figure  repre- 
sents a  forcing-pump,  with  a  solid 
plunger  c,  and  a  valve  n  that  opens        Fig.  36. 


244  HYDROSTATICS, 
upwards,  through  which  the  water  is  brought  into 
the  pipe  n  o.  By  bringing  down  the  plunger  r,  the 
water  in  n  o  is  forced  through  the  valve  x  into  the 
bottom  of  the  cylinder,  and  thereby  drives  up  the 
plunger  e. 

C.  What  does  m  represent  ? 

F.  A  bundle  of  hay,  or  bag  of  cotton,  or  any  other 
substance  that  it  may  be  desirable  to  bring  into  a 
compass  twenty  or  thirty  times  less  than  it  generally 
occupies. 

E,  I  see  now  the  whole  operation  :  the  more  water 
there  is  forced  into  o,  the  higher  the  plunger  is  lifted 
up,  by  which  the  substance  m  is  brought  into  a 
smaller  space. 

jP.  Every  time  the  handle  s  is  lifted  up  the  water 
rushes  in  from  the  well  or  cistern,  and  when  it  is 
brought  down  the  water  must  be  forced  into  the 
cylinder.  The  power  of  this  engine  is  only  limited 
by  the  strength  of  the  materials  of  which  it  is  made, 
and  by  the  force  applied  to  it. 

Mr.  Walker  says,  a  single  man,  working  at  s,  can, 
by  a  machine  of  this  kind,  bring  hay,  cotton  &c.  into 
twenty  times  less  compass  than  it  was  before ;  con- 
sequently a  vessel  carrying  light  goods  may  be  made 
to  contain  twenty  times  more  package  by  means  of 
the  water-press  than  it  could  without  its  assistance. 


PNEUMATICS. 


CONVERSATION  I. 
OF  THE   NATURE   OF  AIR. 

FATHER  CHARLES  EMMA. 

Father,  That  branch  of  natural  philosophy  which 
is  called  Pneumatics  treats  of  the  nature,  weight, 
pressure,  and  spring  of  the  air  which  we  breathe,  and 
of  the  several  effects  dependent  upon  these  properties. 

Charles.  You  told  us  a  few  days  ago,  that  the  air, 
though  to  us  invisible,  is  a  fluid  ;  but  it  surely  differs 
very  much  from  those  fluids  which  you  conversed 
upon  when  treating  of  Hydrostatics. 

F.  It  does  so  ;  but  recollect  the  terms  by  which 
we  defined  a  fluid. 

C.  You  distinguished  a  fluid  as  a  body,  the  parts  of 
which  yield  to  the  least  pressure. 

F.  The  air  in  which  we  live  and  move  will  answer 
to  this  definition  ;  since  we  are  continually  immersed 
in  it,  as  fish  are  in  the  water,  if  the  parts  did  riot  yield 
to  the  least  force,  we  should  be  constantly  reminded  of 
its  presence  by  the  resistance  made  to  our  bodies  ; 
whereas  persons  unaccustomed  to  think  on  these  sub- 
jects are  not  even  aware  that  they  are  surrounded  with 
a  fluid,  the  weight  and  pressure  of  which,  if  not 
counterbalanced  by  ^ome  other  power,  would  in- 
stantly crush  the  human  frame. 

E.  In  a  still  calm  day,  such  as  the  present  is, 
when  one  can  scarcely  discern  a  single  leaf  in  mo- 
tion, it  is  difficult  to  conceive  of  the  existence  of  such 
a  fluid  ;  but  when 

Down  at  once 
Precipitant,  descends  a  mingled  mass 
Of  roaring  winds,  and  flames,  and  rushing-  floods, 

(  Thomson's  Summer) 


246 


PNEUMATICS. 


no  doubi  can  remain  as  to  the  existence  of  some 
mighty  unseen  power. 

C.  By  this  quotation,  Emma,  you  take  it  for 
granted  that  the  air  and  the  winds  are  the  same. 

F,  This  is  really  the  fact,  as  we  shall  prove  On  a 
future  day. 

C.  But  I  am  not  quite  satisfied  that  the  air  is  such 
a  body  as  you  have  described. 

F.  I  do  not  wish  to  proceed  a  single  step  till  1  have 
made  your  mind  easy  upon  this  head. — You  see  how 
easily  those  gold  and  silver  fish  move  in  the  water  : 
can  you  explain  the  reason  of  it  1 

C,  Is  it  not  by  the  exertion  of  their  fins  1 

F,  A  fish  swims  by  the  help  of  his  fins  and  tail ; 
and  fish  in  general  are  nearly  of  the  same  specific 
gravity  with  water.  Take  away  the  water  from  the 
vessel,  and  the  fish  would  still  have  the  use  of  their 
fins  and  tail,  at  least  for  a  short  period. 

E.  And  they  would  flounder  about  at  the  bottom. 

F.  Now  consider  the  case  of  birds,  how  they  fly ; 
the  swallow,  for  instance,  glides  as  smoothly  along  in 
the  air  as  fish  do  in  the  water.  But  if  1  were  to  put 
a  bird,  or  even  a  butterfly,  under  a  glass  receiver, 
however  large,  and  take  away  the  air,  they  would 
have  no  more  use  of  their  wings,  than  fish  have  of 
their  fins  when  out  of  water.  You  shall  see  the  ex- 
periment in  a  day  or  two. 

E.  And  would  they  die  in  this  situation  as  fish  die 
when  taken  from  their  natural  element,  the  water  ? 

jP.  The  cases  are  precisely  similar :  some  fish,  as 
the  carp,  the  eel,  and  almost  all  kinds  of  shell-fish, 
will  live  a  considerable  time  out  of  water ;  so  some 
creatures  which  depend  upon  air  for  existence  will 
live  a  long  time  in  an  fexhausted  receiver  ;  a  butterfly, 
for  instance,  v^ll  fall  to  the  bottom  apparently  lifeless, 
but  admit  the  air  into  the  receiver,  and  it  will  revive  ; 
whereas  experiments  have  been  made  on  mice,  rats, 
birds,  rabbits,  &c.  and  it  is  found  that  they  will  live 
without  air  but  a  very  few  minutes. 

E.  These  are  very  cruel  experiments. 


OF  THE  USES  OF  AIR.  247 
F.  And  ought  by  no  means  to  be  indulged  in  ; 
they  can  be  only  justified  upon  the  presumption,  that 
io  the  hands,  and  under  the  direction,  of  able  philoso- 
phers, they  may  lead  to  discoveries  of  importance  to 
the  health  and  happiness  of  the  human  race. 

C.  Can  fish  live  in  water  from  which  the  air  is 
wholly  excluded  ?  ,  •  • 

F.  The  air  is,  in  fact,  as  necessary  to  their  exis- 
tence, as  it  is  to  ours.  Besides  their  fins,  fish  have 
the  use  of  an  air-vessel,  which  gives  them  full  com- 
mand of  their  various  motions  in  all  depths  of  water, 
which  their  fins  without  it  would  not  be  equal  to. 

E.  What  do  you  mean  by  an  air-vessel  1 

F,  It  is  a  small  bladder  of  air,  so  disposed  within 
them,  that,  by  the  assistance  of  their  muscles,  they 
are  able  to  contract  or  dilate  it  at  pleasure.  By 
contraction  they  become  specifically  heavier  than  the 
water,  and  sink ;  by  dilatation  they  are  lighter,  and 
rise  to  the  surface  more  readily. 

C.  Are  these  opeiations  effected  by  the  external 
air  1  ,  . 

F.  Very  much  so  ;  for  if  you  take  away  the  air 
from  the  water  in  which  a  lish  is  swimrning,  it  will 
no  longer  have  the  power  of  contracting  the  air- 
vessel  within,  which  will  then  become  so  expanded 
as  to  keep  it  necessarily  on  the  surface  of  the  water, 
evidently  to  its  great  inconvenience  and  pam,  and  if 
the  air-bladder  be  pricked  or  broken,  the  fish  pre- 
sently sinks  to  the  bottom,  unable  either  to  support  or 
raise  itself  up  again.  Flat  fish,  as  soles,  plaise,  tur- 
bot,  &c.  have  no  ajr-bladder. 


CONVERSATION  II. 

OF  THE  AIR-PUMP. 

E.  You  told  US,  papa,  of  taking  away  the  air  from 
vessels  ;  will  you  shew  us  how  that  is  performed  1 

F.  I  will ;  and  I  believe  it  will  be  the  most  con- 


248 


PNEUMATICS. 


vincing  method  of  proving  to  you  that  the  air  is  such 
a  body  as  1  have  described. 


Fig.  I- 


This  instrument  is  called  an  air-pump,  and  its  use 
is  to  exhaust  the  air  from  any  vessel,  as  the  glass 
receiver  lk. 

C.  Does  it  act  like  the  common  pump  ? 

F.  So  much  so,  that  if  you  comprehend  the  nature 
and  structure  of  the  one,  you  will  find  but  little  diffi- 
culty in  understanding  the  other.  I  will,  however, 
describe  the  different  parts,  a  a  are  two  strong  brass 
barrels,  within  each  of  which,  at  the  bottom,  is  fixed 
a  valve,  opening  upwards ;  these  valves  communicate 
with  a  concealed  pipe  that  leads  to  k.  The  barrels 
include  also  moveable  pistons,  with  valves  opening 
upwards.* 

E.  How  are  they  moved  ? 

F.  To  the  upper  parts  of  the  pistons  are  attached 
rack-work,  part  of  which  you  see  at  cc  :  these  racks 
are  moved  up  and  down  by  means  of  a  little  cog- 
wheel, turned  round  by  the  handle  r. 

C.  You  turn  the  handle  but  half  way  round. 
F.  And  by  so  doing,  you  perceive  that  one  of  the 
racks  rises  and  the  other  descends. 

*  The  reader  is  supposed  to  have  attended  to  the  struc- 
ture of  the  common  pump,  described  in  Conversation 
XXI.  of  Hydrostatics. 


OF  THE  AIR-PUMP. 

E.  What  is  the  use  of  the  screw  v  1 

F.  It  serves  to  re-admit  air  into  the  receiver  when 
it  is  in  a  state  of  exhaustion,  for  without  such  a  con- 
trivance, the  receiver  could  never  be  moved  out  of 
its  place,  after  the  air  was  once  taken  from  beneath 
it.  But  you  shall  try  for  yourselves.  I  first  place  a 
slip  of  wet  leather  under  the  edge  of  the  receiver,  be- 
cause the  brass  plate  is  liable  to  be  scratched,  and 
the  smallest  unevenness  between  the  receiver  and 
plate  would  prevent  the  success  of  our  experiment. 
—I  have  turned  the  handle  but  a  few  times  :  try  to 
take  away  the  receiver. 

C.  I  cannot  move  it. 

F.  I  dare  say  not ;  for  now  the  greater  part  of  the 
air  is  taken  from  under  the  receiver,  and  consequently 
it  is  pressed  down  with  the  weight  of  the  atmosphere 
on  the  outside. 

E.  Pray  explain  how  the  air  was  taken  away. 

F.  By  turning  the  winch  r  half  way  round  I  raise 
one  of  the  pistons,  and  thereby  leave  a  vacuum  in  the 
lower  part  of  the  barrel,  and  a  portion  of  the  air  in 
the  receiver  rushes  through  the  pipe  into  the  empty 
barrel.  I  then  turned  the  winch  the  other  way, 
which  raised  the  other  piston,  and  a  vacuum  would  be 
left  in  that  barrel,  did  not  another  portion  of  air 
rush  from  the  receiver  into  it.  .  . 

C.  When  the  first  piston  descended,  did  the  air  in 
the  barrel  open  the  little  valve,  and  escape  by  the 

^^V.^It  did ;  and  by  the  alternate  working  of  the 
pistons,  so  much  of  the  air  is  taken  away,  that  the 
quantity  left  has  not  force  enough  to  raise  the  valve. 

C.  Cannot  you  take  all  the  air  from  the  receiver  ? 

F,  Not  by  means  of  the  air-pump. 

E.  What  is  the  reason  that  a  mist  comes  on  the 
inside  of  the  glass  receiver  while  the  air  is  exhausting  1 

E.  It  is  explained  by  the  sudden  expansion  of  the 
air  that  is  left  in  the  receiver,  which  we  shall  notice 
more  particularly  in  our  Conversations  on  Chemistry. 
M  2 


250  PNEUMATICS. 

C.  You  have  not  told  us  the  use  of  the  smaller 
receiver  w,  v^^ith  the  bottle  of  quicksilver  within  it. 

F.  By  means  of  the  concealed  pipe  there  is  a  com- 
munication between  this  and  the  large  receiver,  and 
the  whole  is  intended  to  shew  to  what  degree  the  air 
in  the  large  receiver  is  exhausted.  It  is  called  the 
small  barometer-guage,  the  meaning  of  which  you  will 
better  understand  when  the  structure  of  the  barometer 
is  explained. — I  will  now  shew  you  an  experiment  or 
two,  by  which  the  resistance  of  the  air  is  clearly 
demonstrated. 

£.  Are  these  mills  for  the  pur- 
pose ? 

F.  Yes,  they  are  ;  the  machine 
consists  of  two  sets  of  vanes,  a  and  b, 
made  equally  heavy,  and  to  move 
on  their  axes  with  the  same  free- 
dom. 

C.  But  the  vanes  of  a  are  placed 
edgeways,  and  those  of  b  are 
breadthways. 

F,  They  are  so  placed  to  exhibit  in  a  striking 
manner  the  resistance  of  the  atmosphere ;  for  as  the 
little  mill  a  turns,  it  is  resisted  only  in  a  small  de- 
gree, and  will  go  round  a  much  longer  time  than  the 
other,  which,  in  its  revolutions,  meets  the  air  with  its 
whole  surface.  By  means  of  the  spring  c  resting 
against  the  slider  d  in  each  mill,  the  vanes  are  kept 
fixed. 

E.  Shall  I  push  down  the  sliders  ? 

F.  Do  so  :  you  see  that  both  set  off  with  equal 
velocities. 

C.  The  mill  b  is  evidently  declining  in  swiftness, 
while  the  other  goes  on  as  quick  as  ever. 

F,  Not  quite  so ;  for  in  a  few  minutes  you  will 
find  them  both  at  rest. 

Now  we  will  place  them  under  the  receiver  of  the 
air-pump,  and  by  a  little  contrivance  we  shall  be 
able  to  set  the  mills  going  after  the  air  is  exhausted 


1^1 

Fig 

.  2. 

RESISTANCE  OF  THE  AIR.  251 
from  the  receiver,  and  then,  as  there  is  no  sensible 
resistance  against  them,  they  will  both  move  round  a 
considerable  time  longer  than  they  did  in  the  open 
air,  and  the  instant  that  one  stops  the  other  will  stop 
also. 

E.  This  experiment  clearly  shews  the  resisting 
power  of  the  air. 

F,  It  shews  also  that  its  resistance  is  in  proportion 
to  the  surface  opposed  to  it ;  for  the  vane  which  met 
and  divided  the  air  by  the  edge  only,  continued  to 
move  the  longest  while  they  were  both  exposed  to  it ; 
but  when  that  is  removed,  they  both  stop  together, 
because  there  is  nothing  now  to  retard  their  motion 
but  the  friction  on  the  pivots,  which  is  the  same  in 
both  cases.  Take  this  guinea  and  a  feather  j  let  them 
both  drop  from  your  hand  at  the  same  instant. 

C.  The  guinea  is  soon  at  rest  at  my  feet,  but  the 
feather  continues  floating  about.  Is  the  feather  spe- 
cifically lighter  than  air  ] 

F.  No  ;  for  if  it  were,  it  would  ascend  till  it  found 
the  air  no  heavier  than  itself;  whereas  in  a  minute  or 
two,  you  will  see  the  feather  on  the  floor  as  well  as  the 
guinea  :  it  is  however  so  light,  and  presents  so  large 
a  surface,  in  comparison  to  its  weight,  to  the  air,  that 
it  is  considerably  longer  in  falling  to  the  ground  than 
heavier  bodies,  such  as  a  guinea.  Take  away  the 
resisting  medium,  and  they  will  both  reach  the  bottom 
at  once. 

E.  How  will  you  do  that  ?  y| 

F,  Upon  this  brass  flap  I  place  the  guinea  cS^^ 


and  the  feather,  and  having  turned  up  the 
flap  and  shut  it  into  a  small  notch,  I  fix 
the  whole  on  a  tall  receiver,  with  a  piece 
of  wet  leather  between  the  receiver  and 
brass.  I  will  now  exhaust  the  air  from 
under  the  receiver  by  placing  it  over  the 
air-pump,  and  if  I  turn  the  wire/ a  little, 
the  flap  will  slip  down,  and  the  guinea 
and  feather  will  fall  with  equal  velocities. 


252 


PNEUMATICS. 


C.  They  are  both  at  the  bottom,  but  I  did  not  see 
tliem  fall. 

F.  While  I  repeat  the  experiment,  you  must  look 
stedfastly  to  the  bottom,  because  the  distance  is  too 
small  for  you  to  trace  their  motion ;  but  by  keeping 
your  eye  at  the  bottom  you  will  see  the  feather  and 
the  guinea  arrive  at  the  same  instant. 

In  this  glass  tube  is  some  water,  but  the 
air  is  taken  away,  and  the  glass  completely 
closed.  Turn  it  up  quick,  so  that  the  water 
may  fall  on  the  other  end. 

E.  It  makes  a  noise  like  the  stroke  of  a 
hammer. 

F,  And  for  that  reason  it  is  usually  called 
the  philosophical  hammer.     The  noise  is  ^ 
occasioned  through  the  want  of  air  to  break  j--  ^ 
the  fall ;  for  if  I  take  another  glass  in  all 
respects  like  it,  but  having  air  inclosed  in  it  as  well 
as  water,  you  may  turn  it  as  often  as  you  please 
with  hardly  any  noise. 


CONVERSATION  III. 

OF  THE  TORRICELLIAN  EXPERIMENT. 

C.  If  by  means  of  the  air-pump  you  cannot  per- 
fectly exhaust  the  air  from  any  vessel,  by  what  means 
is  it  done  1 

F.  This  glass  tube  is  about  36  inches  long,  and 
open  at  one  end  only.  I  fill  it  very  accurately  with 
quicksilver,  and  placing  my  thumb  over  the  open  end, 
I  invert  the  tube,  and  plunge  it  into  a  vessel  of  the 
same  metal,  taking  care  not  to  remove  my  thumb  till 
the  end  of  the  tube  is  completely  immersed  in  the 
quicksilver. — You  observe  the  mercury  is  suspended 
in  the  tube  to  a  certain  height,  and  above  it  there  is 
a  perfect  vacuum  ;  that  is,  in  the  six  or  seven  inches 
at  the  upper  part  of  the  tube  the  air  is  perfectly  ex- 
cluded. 


TORRICELLIAN  EXPERIMENT.  253 

E.  Could  not  the  air  get  in  when  you  took  away 
your  thumb  ? 

F.  You  saw  that  I  did  not  remove  my  thumb  till 
the  open  end  of  the  tube  was  wholly  under  the  quick- 
silver, therefore  no  air  could  get  into  the  tube  without 
first  descending  through  the  quicksilver :  now  you 
know  that  a  lighter  fluid  will  not  descend  through  one 
that  is  heavier,  and  consequently  it  is  impossible  that 
any  air  should  be  in  the  upper  part  of  the  tube. 

C.  What  makes  the  quicksilver  stand  at  that  par- 
ticular height  I 

F.  Before  I  answer  this,  tell  me  what  is  the  reason 
that  water  cannot  be  raised  by  means  of  a  common 
pump  higher  than  about  32  or  33  feet  ? 

C.  Because  the  pressure  of  the  atmosphere  is  equal 
to  the  pressure  of  a  column  of  water  so  many  feet  in 
height.* 

F.  And  the  pressure  of  a  column  of  quicksilver  29 
inches  long,  a  little  more  or  less  according  to  the 
variation  of  the  air,  is  equal  to  the  pressure  of  a 
column  of  water  32  or  33  feet  high,  and  consequently 
equal  to  the  pressure  of  the  whole  height  of  the  atr/io- 
sphere. 

E.  Is  then  the  mercury  in  the  tube  kept  suspended 
bv  the  weight  of  the  air  pressing  on  that  in  the  cup  1 
■'F.  It  is. 

E.  If  you  could  take  away  the  air  from  the  cup, 
would  the  quicksilver  descend  in  the  tube  ? 

F.  If  I  had  a  receiver  long  enough  to  enclose  the 
cup  and  tube,  and  were  to  place  them  on  the  air- 
pump,  you  would  see  the  effect  that  a  single  turn  of 
the  handle  would  have  on  the  mercury  ;  and,  after  a 
few  turns,  the  quicksilver  in  the  tube  would  be  nearly 
on  a  level  with  that  in  the  cup. 

I  can  shew  you  by  means  of  this  syringe,  that  the 
suspension  of  the  quicksilver  in  the  tube  is  owing  to 
nothing  but  the  pressure  of  the  air, 

*  See  Hydrostatics,  CDnversation  XXI. 


254  PNEUMATICS. 

C.  What  Is  the  structure  of  the  syringe  ? 

jP.  If  you  understand  in  what  manner  a  common 
water-squirt  acts,  you  will  be  at  no  loss  about  the 
syringe,  which  is  made  like  it. 

C.  By  dipping  the  small  end  of  a  squirt  in  water, 
and  lifting  up  the  handle,  a  vacuum  is  made,  and  then 
the  pressure  of  the  air  on  the  surface  of  the  water 
forces  it  into  the  squirt. 

F.  That  is  the  proper  explanation. — 
This  vessel  d,  containing  some  quick- 
silver, and  the  small  tube  gf,  33  inches 
long,  open  at  both  ends,  immersed  in  it, 
are  placed  under  a  large  receiver  a  b  ;  the 
brass  plate  c,  put  upon  it  with  a  piece  of 
wet  leather,  admits  the  small  tube  to 
pass  through  it  at  h.  I  will  now  screw 
the  syringe  h  on  the  tube  g  f,  and  by 
lifting  up  the  handle  i,  a  partial  vacuum 
is  made  in  the  tube ;  consequently  the 
pressure  of  the  air  in  the  receiver  upon 
the  mercury  in  the  cup  d  forces  it  up  into  Fig.  5. 
the  little  tube  as  high  as  a\  just  in  the 
same  manner  as  water  follows  the  piston  in  a  common 
pump." 

E.  But  is  not  this  rise  of  the  quicksilver  in  the  tube 
owing  to  the  suction  of  the  syringe  1 

F.  To  prove  to  you  that  it  is  not,  I  place  the  whole 
apparatus  over  the  air-pump,  and  exhaust  the  air  out 
of  the  receiver  a  b.  This  operation,  you  must  be  sensi- 
ble, has  not  the  smallest  effect  on  the  air  in  the 
syringe  and  little  tube  ;  but  you  nevertheless  observe, 
that  the  mercury  has  again  fallen  into  the  cup  d  : 
and  the  syringe  might  now  be  worked  for  ever  without 
raising  the  mercury  in  the  tube ;  but  admit  the  air 
into  the  receiver,  and  its  action  upon  the  surface  of 
the  quicksilver  in  the  cup  will  force  it  instantly  into 
the  tube. 

This  is  called  the  Torricellian  experiment,  in  honour 
of  Tonicelli,  a  learned  Italian,  and  disciple  of  Ga- 


OF  THE  PRESSURE  OF  THE  AIR.  255 

lileo,  who  invented  it ;  who  was  the  first  person  that 
discovered  the  pressure  and  weight  of  the  air,  and  the 
father  of  all  modern  discoveries  respecting  the  proper- 
ties of  the  atmospheric  air. 


CONVERSATION  IV. 

OF  THE  PRESSURE   OF  THE  AIR. 

C.  It  seems  very  surprising  that  the  air,  which  is 
invisible,  should  produce  such  effects  as  you  have 
described. 

F,  If  you  are  not  satisfied  with  the  evidence 
which  your  eyes  are  capable  of  affording,  you  would, 
perhaps,  have  no  objection  to  the  information  which 
your  feelings  may  convey  to  your  mind. 
Place  this  little  glass  a  b,  open  at  both 
ends,  over  the  hole  of  the  pump  plate, 
and  lay  your  hand  close  upon  the  top  b, 
while  1  turn  the  handle  of  the  pump  a  few 
times. 

C.  It  hurts  me  very  much  :  I  cannot  take  my 
hand  away. 

F.  By  letting  in  the  air,  I  have  released  you.  The 
pain  was  occasioned  by  the  pressure  of  the  air  on  the 
outside  of  your  hand,  that  being  taken  away  from 
under  it  which  served  to  counterbalance  its  weight. 

This  is  a  larger  glass  of  the  same  kind  ; 
over  the  large  end  I  tie  a  piece  of  wet 
bladder  very  tight,  and  will  place  it  on 
the  pump,  and  take  the  air  from  under  it. 

E.  Is  it  the  weight  of  the  air  that  bends 
the  bladder  so  much  1  Fig.  7. 

F.  Certainly  :  and  if  I  turn  the  handle 
a  few  more  times  it  will  burst. 

C.  It  has  made  a  report  as  loud  as  a  gun. 

F.  A  piece  of  thin  flat  glass  may  be  broken  in  the 


256 


PNEUMATICS. 


Fio-.  8. 


same  manner. — Here  is  a  glass  bubble  a 
with  a  long  neck,  which  I  put  into  a  cup 
of  water  b,  and  place  them  under  a  re- 
ceiver on  the  plate  of  the  air-pump,  and 
by  turning-  the  handle  the  air  is  not  only 
taken  from  the  receiver,  but  that  in  the 
hollow  glass  ball  will  make  its  way 
through  the  water  and  escape. 

E.  Is  it  the  air  which  occasions  the  bubbles  at  the 
surface  of  the  water  ? 

F.  It  is.  And  now  the  bubbling  is  stopped,  and 
therefore  I  know  that  as  much  of  the  air  is  taken 
away  as  can  be  got  out  by  means  of  the  pump.  The 
hollow  ball  is  still  empty  :  and  by  turning  the  cock  v 
of  the  pump  (Fig.  1.)  the  air  rushes  into  the  receiver 
and  presses  upon  the  water,  thereby  filling  the  ball 
with  the  fluid. 

C.  It  is  not  quite  full. 

F.  That  is  because  the  air  could  not  be  perfectly 
exhausted,  and  the  little  bubble  of  air  at  the  top  is 
what,  in  its  expanded  state,  filled  the  whole  glass 
ball,  and  now  by  the  pressure  of  the  external  air  is  re- 
duced into  the  size  you  see  it. 

Another  very  simple  experiment 
will  convince  you  that  suction  has 
nothing  to  do  with  these  experiments. 
On  the  leather  of  the  air-pump  at  a 
little  distance  from  the  hole,  I  place 
lightly  this  small  receiver  .r,  and  pour 
a  spoonful  or  two  of  water  round  the 
edge  of  it.  I  now  cover  it  with  a 
larger  receiver  a  b,  and  exhaust  the 
air. 

E,  I  see  by  the  bubbles  round  the  edge  of  the 
small  receiver  that  the  air  is  making  its  way  from 
under  it. 

F.  I  have  pretty  well  exhausted  all  the  air.  Can 
you  move  the  large  receiver  1 

C.  No  ;  but  by  shaking  the  pump,  I  see  the  little 
one  is  loose. 


TW.  9. 


OF  THE  PRESSURE  OF  THE  AIR.  257 
F.  The  large  one  is  rendered  immoveable  by  the 
pressure  of  the  external  air.    But  the  air  being  taken 
from  the  inside  of  both  glasses,  there  is  nothmg  to 
fasten  down  the  smaller  receiver. 

E.  But,  if  suction  had  any  thing  to  do  with  this 
business,  the  little  receiver  would  be  fast  as  well  as 
the  other. 

F,  Turn  the  screw  v  of  the  air-pump  (Fig.  1.) 
quickly.    You  hear  the  air  rushing  in  with  violence. 

C.  And  the  large  receiver  is  loosened  again. 
F.  Take  away  the  smaller  one,  Emma. 

E.  I  cannot  move  it  with  all  my  strength. 

F,  Nor  could  you  lift  it  up  if  you  were  a  hundred 
times  stronger  than  you  are.  For  by  admitting  the 
air  very  speedily  into  the  large  receiver  it  pressed 
down  the  little  one  before  any  air  could  get  under- 
neath it. 

C.  Besides,  I  imagine  you  put  the  water  round  the 
edge  of  the  glass  to  prevent  the  air  from  rushing  be- 
tween it  and  the  leather. 

F.  You  are  right ;  for  air,  being  the  lighter  fluid, 
could  not  descend  through  the  layer  of  water  in 
order  to  ascend  into  the  receiver. — Could  suction 
produce  the  effect  in  this  experiment  ? 

C.  I  think  not :  because  the  little  receiver  was  not 
fixed  till  after  what  might  be  thought  suction  had 
ceased  to  act. 

F,  Kight :  and  to  impress  this  fact  strongly  on 
your  mind,  I  will  repeat  the  experiment ;  you  observe 
that  the  air  being  taken  from  under  both  receivers, 
the  large  one  must  be  fixed  by  the  pressure  of  the 
atmosphere,  and  the  smaller  one  must  be  loose,  be- 
cause there  is  no  pressure  on  its  outside  to  fasten  it. 
But  by  admitting  the  air,  the  inner  one  becomes  fixed 
by  the  very  means  that  the  outer  one  is  loosened. 

£.  How  will  you  get  the  small  one  away  1. 

F.  As  I  cannot  raise  it,  I  must  slide  it  over  the  hole 
in  the  brass  plate  ;  and  now  the  air  gets  under  it, 
there  is  not  the  smallest  difficulty  ;  otherwise,  it 
could  scarcely  be  lifted  by  the  strength  of  any  one. 


258 


PNEUMATICS. 


CONVERSATION  V. 

OF  THE   PRESSURE   OF  THE  AIR. 

C.  Although  suction  has  nothing  to  do  in  the  ex- 
periments which  you  made  yesterday,  yet  I  think  I 
can  shew  you  an  instance  in  which  it  has.  This 
experiment,  if  such  it  may  be  called,  I  have  made 
a  hundred  times.  1  fasten  a  string  in  the  centre 
of  a  round  piece  of  leather,  and  having  thoroughly 
soaked  it  in  water,  I  press  it  on  a  flat  stone,  and  by 
pulling  at  the  string  the  leather  draws  up  the  stone, 
although  it  be  not  more  than  two  or  three  inches  in 
diameter,  and  the  stone  weighs  several  pounds. 
Surely  this  is  suction. 

J^.  I  should  say  so  too  if  I  could  not  account  for  it 
by  the  pressure  of  the  atmosphere.  By  pressing  the 
wet  leather  on  the  stone  you  displace  the  air,  then  by 
pulling  the  string  a  vacuum  is  left  at  the  centre,  and 
the  pressure  of  the  air  about  the  edges  of  the  leather 
is  so  great,  that  it  requires  a  greater  power  than  the 
gravity  of  the  stone  to  separate  them. 

I  have  seen  you  drink  water  from  a  spring  by 
means  of  a  hollow  straw. 

E.  Yes,  that  is  another  instance  of  what  we  have 
been  accustomed  to  call  suction. 

F.  But  now  you  know,  that  in  this  operation  you 
make  a  syringe  with  the  straw  and  your  lips,  and  by 
drawing  in  your  breath  you  cause  a  vacuum  in  the 
hollow  straw  tube,  and  the  pressure  of  the  air  on  the 
water  in  the  spring  forces  it  up  through  the  straw 
into  the  mouth. 

C.  I  cannot,  however,  help  thinking  that  this  looks 
like  suction,  for  the  moment  I  cease  the  drawing  in  my 
breath,  the  Water  ceases  to  rise  in  my  mouth. 

F,  That  is,  when  there  is  no  longer  a  vacuum  in 
the  straw,  the  pressure  within  is  just  equal  to  that 
without,  and  consequently  the  water  will  rest  at  its 
natural  level. 


OF  THE  TRANSFERRER. 


259 


I  will  shew  you  anothei' 
striking  instance  of  the 
efFects  of  the  air's  pressure. 
This  instrument  is  called 


the  transferrer.  The  screw  gm 
c  fits  on  to  the  plate  of  the  «^ 


air-pump,  and  by  means  of  'J^\ 
the  stop-cocks  g  and  h,  I  ^ 
can  take  away  the  air  from 
both,  or  either,  of  the  re-  Fig.  10. 

ceivers,  i  k,  at  pleasure. 

E.  Is  there  a  channel  then  running  from  c  through 
DAB,  and  thence  passing  to  the  receivers?  , 

F.  There  is.  I  will  screw  the  whole  on  the  air- 
pump,  and  turn  the  cock  g,  so  that  there  is  now  no  com- 
munication from  C  to  the  internal  part  of  the  receiver 
I.  At  present  you  observe  that  both  the  receivers  are 
perfectly  free.  By  turning  the  handle  of  the  pump 
a  few  times  the  air  is  taken  away  from  the  receiver  k, 
and  to  prevent  its  re-entrance  I  turn  the  stop-cock  d. 
Try  if  you  can  move  it. 

C.  I  cannot ;  but  the  other  is  loose. 

F.  The  pressure  of  the  atmosphere  is  evidently  the 
same  on  the  two  receivers  ;  but  with  regard  to  i,  the 
pressure  within  is  equal  to  that  without,  and  the  glass 
is  free  ;  in  the  other,  the  pressure  from  withm  is  taken 
away,  and  the  glass  is  fixed.  In  this  stage  of  the 
experiment  you  are  satisfied  that  there  is  a  vacuum  m 
the  receiver  k.  By  turning  the  cock  g,  I  open  a 
communication  between  the  two  receivers,  and  you 
hear  the  air  that  was  in  i  rush  through  the  channel 
A  B  into  K.  Now  try  to  move  the  glasses. 

E.  They  are  both  fixed  :  how  is  this  1. 

F.  The  air  that  was  inclosed  in  the  glass  i  is 
equally  diflfused  between  the  two,  consequently  the 
internal  pressure  of  neither  is  equal  to  the  external, 
and  therefore  they  are  both  fixed  by  the  excess  of  the 
external  pressure  over  the  internal.  In  this  case  it 
could  not  be  suction  that  fixed  the  glass  i,  for  it  was 
free  long  after  what  might  have  been  thought  suction 
had  ceased  to  act. 


260  PNEUMATICS. 

C.  What  are  these  brass 
cups?  /J^X 

F,  They  are  called  the 
hemispherical  cups  ;  I  will 
bring  the  two,  b  a,  together, 
with  a  wet  leather  between 
them,  and  then  screw  them 
by  D  to  the  plate  of  the  air- 
pump  :  and  having  exhaust- 
ed the  air  from  the  inside,  I 
turn  the  stop-cock  e,  take  them  from  the  pump,  and 
screw  on  the  handle  f.  See  if  you  two  can  separate 
them. 

E,  We  cannot  stir  them. 

F .  If  the  diameter  of  these  cups  were  four  inches, 
the  pressure  to  be  overcome  would  be  equal  to  1801b! 
I  will  now  hang  them  up  in  the  receiver  (Fig.  12.) 
and  exhaust  the  air  out  of  it,  and  you  see  they 
separate  without  the  application  of  any  force. 

C.  Now  there  is  no  pressure  on  the  outside,  and 
therefore  the  lower  cup  falls  off  by  its  own  gravity. 

-F.  With  this  steel-yard  you 
may  try  very  accurately  to  what 
weight  the  pressure  of  the  atmos- 
phere against  the  cups  is  equal.* 

E,  For  when  the  weight  w  is 
carried  far  enough  to  overcome 
the  pressure  of  the  cups,  it  lifts 
up  the  top  one. 

F.  I  have  exhausted  the  air  of 
this  receiver  h,  consequently  it  is 
fixed  down  to  the  brass  plate  i ; 
to  the  plate  is  joined  a  small  tube 
with  a  stop-cock  x ;  by  placing 
the  lower  end  of  the  tube  in  a 
bason  of  water,  and  turning  the 
cock,  the  pressure  of  the  atmos- 
phere on  the  water  in  the  bason 
forces  it  through  the  tube  in  the 
form  of  a  fountain.  This  is  called 
the  fountain  in  vacuo.  Fio-.  14. 


Fi^.  15. 


OF  THE  WEIGHT  OF  AIR. 

To  this  little  square  bottle  a  h 

is  cemented  a  screw  valve,  by 

vi^hich  I  can  fix  it  on  the  plate  of 

the  air-pump,  and  exhaust  its  air : 

and  you  will  see  that  when  there 

is  no  power  within  to  support  the 

pressure  of  the  atmosphere  from 

without,  it  will  be  broken  into  a 

thousand  pieces. 

C.  Why  did  you  not  use  a  round  phial  1 

F.  Because  one  of  that  shape  would  have  sustained 

the  pressure  like  an  arch. 

E.  Is  that  the  reason  why  the  glass  receivers  are 
able  to  bear  such  a  weight  without  breaking  1 

F.  It  is :  if  mercury  be  poured  into  a 
wooden  cup  c,  made  of  willow,  and  the 
air  taken  from  under  it,  the  mercury  will, 
by  the  weight  of  the  external  air,  be 
forced  through  the  pores  of  the  wood, 
and  descend  like  a  shower  of  rain.  This 
experiment  proves,  satisfactorily,  the  great 
pressure  of  the  atmosphere. 


Fig.  16, 


CONVERSATION  VI. 

OF  THE  WEIGHT  OF  AIR. 

E.  We  have  seen  the  surprising  effects  of  the  air's 
pressure  ;  are  there  any  means  of  obtaining  the  exact 
weight  of  air  ? 

F.  If  you  do  not  require  any  very  great  nicety,  the 
method  is  very  simple. 

*  The  principle  of  the  steel-yard  (referred  to  p.  260) 
is  explained  in  Conversation  XV.  of  Mechanics. 


2G2 


PNEUMATICS 


This  Florence  flask  is  fitted 
up  with  a  screw,  and  a  fine  oiled 
silk  valve  at  d.  I  will  now 
screw  the  flask  on  the  plate  of 
the  air-pump,  and  exhaust  the 


air.    You  see  in  its  present  ex-  /  „  i 

hausted  state  it  weighs  3  ounces 
and  5  grains.  Fig.  17. 

C.  Cannot  the  air  get  through 
the  silk? 

F,  The  silk,  being  varnished  with  a  kind  of  oily 
substance,  is  impenetrable  to  air  ;  and,  being  ex- 
hausted, the  pressure  upon  the  outside  effectually 
prevents  the  entrance  of  the  air  by  the  edges  of  the 
silk  ;  but  if  I  lift  it  up  by  means  of  this  sewing  needle, 
you  will  hear  the  air  rush  in. 

E,  Is  that  hissing  noise  occasioned  by  the  re- 
entrance  of  the  air  1 

F,  It  is  ;  and  when  that  ceases  you  may  be  sure 
the  air  within  the  bottle  is  of  the  same  density  as  that 
without. 

C.  If  I  weigh  it  again,  the  difference  between  the 
weight  now,  and  when  you  tried  it  before,  is  the 
weight  of  the  quantity  of  air  contained  in  the  bottle  : 
— it  weighs  very  accurately  3  ounces  19-  grains,  con- 
sequently the  air  weighs  14|  grains. 

F.  And  the  flask  holds  a  quart,  wine  measure. 

E,  Does  a  quart  of  air  always  weigh  14 j  grains  1 

F.  The  weight  of  the  air  is  perpetually  changing  ; 
therefore  though  a  quart  of  it  weighs  to-day  14~ 
grains,  the  same  quantity  may,  in  a  few  hours,  weigh 
14§  grains,  or  perhaps  only  14  grains,  or  more  or 
less.  The  air  is  much  heavier  this  morning  than  it 
was  at  the  same  time  yesterday. 

C.  How  do  you  know  that ;  did  you  weigh  some 
yesterday  1 

F,  No  ;  but  the  rising  and  falling  of  the  quick- 
silver in  the  barometer,  an  instrument  which  1  shall 
hereafter  very  particularly  describe,  are  sure  guides  by 


OF  THE  WEIGHT  OF  AIR.  203 
which  the  real  weight  of  ihe  air  is  estimated ;  and  it 
stands  full  three-tenths  of  an  inch  higher  now  than  it 
did  yesterday. 

E.  Will  you  explain  how  we  may  judge  ot  the 
different  weights  of  the  air  by  the  barometer  1 

F.  This  subject  might,  perhaps,  be  better  discussed 
when  we  come  to  treat  explicitly  on  that  instrument ; 
but  I  will  now  answer  your  inquiry,  although  I 
should  be  in  some  danger  of  a  repetition  on  a  future 
day. 

I      The  mercury  in  a  well-made  barometer  will  always 
1  subside  till  the  weight  of  the  column  be  exactly 
equivalent  to  the  weight  of  the  external  air  upon  the 
:  surface  of  the  mercury  in  the  bason,  consequently 
!  the  height  of  the  mercury  is  a  sure  criterion  by 
f  which  that  weight  is  to  be  estimated.— Suppose,  for 
example,  the  barometer  stands  at  29J  inches,  or,  as 
it  is  usually  expressed,  at  29.5,  and  I  find  a  quart  of 
air  at  that  time  weighs  14|  grains.    Here  then  is  a 
,  standard  by  which  I  may  ever  after  compare  the 
gravity  of  the  atmosphere.    If  to-morrow  1  find  the 
!  quicksilver  has  fallen  to  29.3,  I  shall  know  the  air  is 
not  so  heavy  as  it  was  ;  because,  in  this  case  a  column 
of  quicksilver,    29.3   inches,    balances  the  whole 
'  weight;  whereas  it  required  before  a  column  equal 
!  to  29.5.    If,  on  the  contrary,  when  I  look  again,  the 
ii  mercury  has  risen  to  30.7,  as  it  really  stands  at  this 
!  hour,   I  am  sure  the  atmosphere  is  considerably 
I  heavier  than  it  was  before,  and  that  a  quart  of  it  will 
\  weigh  more  than  14|  grains. 

i      C.  You  intimated,  that  in  weighing  air  the  flask 
j  could  not  be  depended  upon  if  great  nicety  were  re- 
I  quired  ;  what  is  the  reason  of  that  1 
'      F.  I  told  you  when  explaining  the  operations  of 

the  air-pump,  that  it  was  impossible  to  obtain  by 
I  means  of  that  instrument  a  perfect  vacuum.  The 
i  want  of  accuracy  in  the  flask  experiment  depends  on 
1  the  small  quantity  of  air  that  is  left  in  the  vessel  after 
•   the  exhaustion  is  carried  as  far  as  it  will  go  :  this, 

however,  if  the  pump  be  good,  will,  after  12  turns  of 


204  PNEUMATICS. 

the  handle,  be  less  than  the  4000th  part  of  the  whole 

quantity. 

E.  How  do  you  know  this  ? 

F.  You  seem  unwilling  to  take  any  thing  upon 
my  word ;  and  in  subjects  of  this  kind  you  do 
right  never  to  rest  satisfied  without  a  reason  for  what 
is  asserted. 

I  suppose,  then,  each  of  the  oarrels  of  the  air-pump 
is  equal  in  capacity  to  the  flask  ;  that  is,  each  will 
contain  a  quart ;  then  it  is  evident,  that,  by  turning 
the  handle  of  the  pump,  I  exhaust  all  the  air  of  one 
barrel  and  the  air  in  the  flask  becomes  at  the  same 
time  equally  diffused  between  the  barrel  and  flask  ; 
that  is,  the  quart  is  now  divided  into  two  equal  parts, 
one  of  which  is  in  the  flask  and  the  other  in  the  bar- 
rel. By  the  same  reason,  at  the  next  turn  of  the 
handle,  the  pint  in  the  flask  will  be  reduced  to  half  a 
pint ;  and  so  it  will  go  on  decreasing,  by  taking  away, 
at  every  turn,  one  half  of  the  quantity  that  was  left 
in  by  the  last  turn. 

C.  Do  you  mean,  then,  that  after  the  first  turn  of 
the  handle,  the  air  in  the  bottle  is  twice  as  rare  as  it 
was  at  first ;  and  after  the  second,  third,  and  fourth 
turns,  it  is  four  times,  eight  times,  and  sixteen  times 
as  rare  as  it  was  when  you  beganl 

F,  That  is  what  I  meant ;  carry  on  your  multipli- 
cation, and  you  will  find  that  after  the  twelfth  turn  it 
is  4096  times  more  rare  than  it  was  at  first. 

E.  I  now  understand,  that  though  absolute  exact- 
ness be  not  attainable,  yet  in  weighing  this  quart  of 
air,  the  error  is  only  equal  to  the  4096th  part  of  the 
whole,  which  quantity  may,  in  reasoning  on  the  sub- 
ject, be  overlooked. 

F.  I  will  now  exhaust  the  flask  again  of  its  air,  and 
putting  the  neck  of  it  under  water,  I  will  lift  up  the 
silk  valve,  and  fill  it  with  water.  Now  dry  the  out- 
side very  thoroughly,  and  weigh  it. 

C.  It  weighs  27  ounces. 

F.  Subtract  the  weight  of  the  flask,  and  reduce  the 
remainder  into  grains,  and  divide  by  14^,  and  you 


OF  THE  ELASTIGM  Y  OF  AIR.  2fK) 
will  obtain  the  specixic  gravity  of  water  compared 
with  that  of  air. 

C.  I  have  done  it,  and  the  water  is  something  more 
than  800  times  heavier  than  air. 

-F.  Since,  then,  the  specific  gravity  of  water  is 
always  put  at  1,  that  of  air  must  be  as  1  eight- 
hundredth,  af  least  according  to  this  calculation ; 
but  following  the  more  accurate  experiments  of  Mr. 
Cavendish  and  others,  whose  authority  may  be  safely 
appealed  to,  the  specific  gravity  of  air  at  the  surface 
of  the  earth  is  800  times  less  than  that  of  water,  when 
the  barometer  stands  as  high  as  30  inches. 

Now  tell  me  what  the  air  in  this  room  weighs, 
which  room  is  25  feet  long,  lO^  high,  and  12|  wide. 

E.  By  multiplying  these  three  numbers  together, 
the  answer  is  3281.25;  so  that  the  room  contains 
rather  more  than  3281  cubic  feet :  the  weight  of  a  cubic 
foot  of  water  is  1000  ounces,  therefore  the  roomful  of 
water  would  weigh  3,281,000  ounces  ;  but  as  air  is  800 
times  lighter  than  water,  the  air  in  the  room  will 
weigh  3,281,000  800  =  4101oz.  =r  2561bs.  5oz. 
It  appears  surprising  that  the  invisible  air  should 
weigh  so  much,  but  as  the  computation  is  m.ade  on 
careful  experiments,  the  fact  cannot  be  doubted. 


CONVERSATION  VII. 

OF  THE  ELASTICITY  OF  AIR. 

F.  I  have  told  you  that  air  is  an  elastic  fluid. 
Now  it  is  the  nature  of  all  elastic  bodies  to  yield 
to  pressure  and  to  endeavour  to  regain  their  former 
figure  as  soon  as  the  pressure  is  taken  oflf.  In  pro- 
jecting an  arrow  from  your  bow,  you  exert  your 
strength  to  bring  the  two  ends  nearer  together,  but 
the  moment  you  let  go  the  string,  it  recovers  its  for- 
mer shape ;  the  power  by  which  this  is  effected  is 
called  elasticity. 

E,  Is  it  not  by  this  power  that  India  rubber,  after 
N 


26G 


PNEUxMATIC3. 


it  has  been  stretched,  recovers  its  usual  size  aruj 
form  1 

F,  It  is :  and  almost  every  thing  that  you  make 
use  of  possesses  this  property  in  a  greater  or  less  de- 
gree :  balls,  marbles,  the  chords  of  musical  instru- 
ments, are  all  elastic. 

C,  I  understand  how  all  these  things  are  elastic  ; 
but  do  not  see  in  what  manner  you  can  prove  the 
elasticity  of  the  air.* 

F,  Here  is  a  bladder,  which  we  will  fill  v/ith  air, 
and  tie  up  its  mouth,  to  prevent  its  escaping  again. 
If  you  now  press  upon  it  with  your  hand,  its  figure 
will  be  changed ;  but  the  moment  the  pressure  is  re- 
moved, it  recovers  its  round  shape. 

E.  And  if  I  throw  it  on  the  ground,  or  against  an^' 
other  obstacle,  it  rebounds,  like  balls  or  marbles. 

F.  You  are  satisfied  also,  1  presume,  that  it  is  the 
air  that  is  the  cause  of  it,  and  not  the  bladder  that 
contains  it. 

Let  us  have  recourse  to  the  air-pump,  to  exhibit 
some  of  the  more  striking  eflfects  of  the  air's  elasticity. 
I  will  let  a  part  of  the  air  out  of  the  bladder,  and  tie 
up  its  mouth  again.  The  pressure  of  the  external  air 
renders  it  flaccid,  and  you  may  make  what  impression 
you  please  upon  it,  without  its  endeavouring  to  re- 
assume  its  former  figure. 

E.  What  proof  is  there  that  this  is  owing  to  the 
external  pressure  of  the  air  ? 

F.  Such  as  will  satisfy  you  both,  I  am  sure.  Place 
it  under  the  receiver  of  the  air-pump,  exhaust  the 
air,  and  see  the  consequences. 

C.  It  begins  to  swell  out ; — and  now  it  is  as  large 
as  when  it  was  blown  out  full  of  air. 

E.  The  outward  pressure  being  in  part  removed, 
the  particles  of  air,  by  their  elasticity,  distend,  and  fill 
up  the  bladder  ;  and  if  it  were  much  larger,  and  the 
exhaustion  were  carried  farther,  the  same  small  quan- 
tity of  air  would  fill  it  completely.  I  will  now  let  the 
air  in  again. 

*  See  Conver.  XIII.  Mechanics. 


OF  THE  ELASTICITY  OF  AIR.  267 

E.  This  exhibits  a  very  striking  proof  of  the  power 
and  pressure  of  the  external  air,  for  the  bladder  is  as 
flaccid  as  it  was  before. 

F.  I  put  the  same  bladder  into  this  square  box 
without  any  alteration,  and  lay  upon  it  a  moveable 
lid,  upon  which  I  place  this  weight.  By  bringing  the 
whole  under  a  receiver,  and  exhausting  the  external 
air,  the  elasticity  of  that  in  the  bladder  will  lift  up 
the  lid  and  weight  together. 

C.  If  you  pump  much  more  the  weight  will  fall 
against  the  side  of  the  glass. 

F.  I  do  not  mean  to  risk  that : — it  is  enough  that 
you  see  a  few  grains,  not  half  a  dozen,  of  air  will,  by 
their  elasticity,  raise  and  sustain  a  weight  of  several 
pounds. 

Take  this  glass  bubble  (see  Fig.  8.)  ;  the  bore  of 
the  tube  is  too  small  for  the  water  to  run  out ;  but  if 
I  place  it  under  the  receiver  of  the  air-pump,  and 
take  away  the  external  air,  the  little  quantity  of  air 
which  is  at  the  top  of  the  glass  will,  by  its  elastic 
force,  expand  itself,  and  drive  out  all  the  water. 

E.  This  experiment  shews,  that  a  very  small  quan- 
tity of  air  is  capable  of  filling  a  large  space,  pro« 
vided  the  external  pressure  is  taken  off. 

F.  Certainly  :  I  will  take  off  the  bladder  from 
this  glass.  (See  Hydrostatics,  Fig.  19.)  The  little 
images  all  swim  at  the  top,  the  air  contained  in  them 
rendering  them  rather  lighter  than  the  water.  Tie 
little  leaden  v/eights  to  their  feet — these  pull  them 
down  to  the  bottom  of  the  vessel :  I  now  place  the 
glass  under  the  receiver  of  the  air-pump,  and  by  ex- 
hausting the  air  from  the  vessel,  that  which  is  within 
the  images,  by  its  elasticity,  expands  itself,  forces  out 
more  water,  and  you  see  they  are  ascending  to  the 
top,  dragging  the  weights  after  them.  I  will  let  in 
the  air,  and  the  pressure  forces  the  water  into  the 
images  again,  and  they  descend. 

Here  is  an  apple  very  much  shrivelled,  which,  Vv^hen 
placed  under  the  receiver,  and  the  external  air  taken 


268  PNEUMATICS. 

away,  will  appear  as  plump  as  if  it  was  newly 

gathered  from  the  tree. 

E.  Indeed  it  now  looks  so  inviting,  that  I  am  ready 
to  wish  it  was  my  own. 

F.  Before,  however,  you  can  get  it,  all  its  beauty 
will  fade.    I  will  admit  the  air  again. 

C.  It  is  as  shrivelled  as  ever.  Do  apples  contain 
air] 

F,  Yes,  a  great  deal,  and  so  in  fact  do  almost 
all  bodies  that  are  specifically  lighter  than  water,  as 
well  as  many  that  are  not  so.  It  was  the  elastic 
power  of  the  air  within  the  apple  that  forced  out  all 
the  shrivelled  parts  when  the  external  pressure  was 
taken  away. 

Here  is  a  small  glass  of  warm  ale,  from  which  I 
am  going  to  take  away  the  air. 

E.  It  seems  to  boil  now  you  exhaust  the  air  from 
the  receiver. 

F.  The  bubbling  is  caused  by  the  air  endeavouring 
to  escape  from  the  liquor.  Let  the  air  in  again,  and 
then  taste  the  beer. 

C.  It  is  flat  and  dead. 

F.  You  see  of  what  importance  air  is  to  give  to  all 
our  liquors  their  pleasant  and  brisk  flavour,  for  the 
same  will  happen  to  wine  and  all  other  fermented 
fluids. 

E.  How  is  it  that  the  air,  when  it  was  re-admitted^ 
did  not  penetrate  the  ale  again  ? 

F.  It  could  not  insinuate  itself  into  the  pores  of  the 
beer,  because  it  is  the  lighter  body,  and  therefore  will 
not  descend  through  the  heavier.  Besides,  it  does  not 
follow  that  it  is  the  same  sort  of  air  which  I  admitted 
into  the  receiver,  that  was  taken  from  the  ale. 

E.  Are  there  more  kinds  of  air  than  one  1 

F,  Yes,  very  many  ;  as  we  shall  shew  you  in  our 
Conversations  on  Chemistry.  That  which  I  took 
from  the  beer,  and  which  gives  it  the  brisk  and  lively 
taste,  is  called  fixed  air,  of  which  there  is  in  general 
but  a  very  small  quantity  in  the  atmosphere. 


OF  THE  ELASTICITY  OF  AIR.  269 

The  elasticity  or  spring  of  air  contained  in  our 
flesh  was  clearly  shewn  by  the  experiment  when  I 
pumped  the  air  from  under  your  hand. 

C.  Was  that  the  cause  of  its  swelling  downward  ? 

F,  It  was  :  and  it  will  account  for  the  pain  you 
felt,  which  was  greater,  and  of  a  very  different  kind, 
than  what  you  would  have  experienced  by  a  dead 
weight  being  laid  on  the  back  of  the  hand,  equal  to 
the  pressure  of  the  air. 

Cupping  is  an  operation  performed  on  this  princi- 
ple ;  the  operator  tells  you  he  draws  up  the  flesh,  but 
if  he  were  to  speak  correctly,  he  would  say  he  took 
away  the  external  air  from  ofl*  the  part  of  the  body, 
and  then  the  elastic  force  of  the  air  within  extends 
and  swells  out  the  flesh  ready  for  his  lancets. 

E,  When  I  saw  you  cupped,  he  did  not  use  an 
air-pump,  but  little  glasses,  to  raise  the  flesh. 

F.  Glasses  closed  at  top  are  now  generally  made 
use  of,  in  which  the  operator  holds  the  flame  of 
a  lamp,  by  the  heat  of  which  the  elasticity  of  the 
air  in  the  glass  is  increased,  and  thereby  a  great  part 
of  it  driven  out.  In  this  state  the  glass  is  put  on  the 
part  to  be  cupped,  and  as  the  inward  air  cools,  it  con- 
tracts, and  the  glass  adheres  to  the  flesh  by  the  difler- 
ence  of  the  pressures  of  the  internal  and  external  air. 

By  some  persons,  however,  the  syringe  is  con- 
sidered as  the  most  eflectual  method  of  performing  the 
operation,  because  by  flame  the  air  cannot  be  rarefied 
more  than  one  half,  whereas  by  the  syringe  a  few 
strokes  will  nearly  exhaust  it. 

Here  is  another  little  square  bottle  like  that  be- 
fore mentioned,  (see  Fig.  15.)  only  that  it  is  full  of 
air,  and  the  mouth  sealed  so  closely  that  none  of  it 
can  escape.  I  enclose  it  within  the  wire  cage  b,  and 
in  this  state  bring  them  under  the  receiver,  and  ex- 
haust the  external  air. 

C.  With  what  a  loud  report  it  has  burst ! 

F,  You  can  easily  conceive  now  in  what  manner 
this  invisible  fluid  endeavours,  continually  by  its 
elastic  force,  to  dilate  itself. 


270  PNEUMATICS. 

E.  Why  did  you  place  the  wire  cage  over  the 
bottle  1 

F.  To  prevent  the  pieces  of  the  bottle  froin  break- 
ing the  receiver,  an  accident  that  would  be  liable  to 
happen  without  this  precaution. 

Take  a  new-laid  egg  and  make  a  small  hole  in  the 
little  end  of  it ;  then,  with  that  end  downwards,  place 
it  in  an  ale-glass  under  the  receiver  and  exhaust  the 
air  ;  the  whole  contents  of  the  egg  will  be  forced  out 
into  the  glass,  by  the  elastic  spring  of  the  small 
bubble  of  air  which  is  always  to  be  found  in  the  large 
end  of  a  new-laid  egg. 


CONVEUSATION  VIII. 

OF  THE  COMPRESSION   OF  THE  AIR. 

F.  1  have  already  alluded  to  the  compressibility  of 
air,  which  it  is  proper  to  describe  here,  it  being  a  cori- 
sequence  of  its  elasticity  ;  for  whatever  is  elastic,  is 
capable  of  being  forced  into  a  smaller  space.  In  this 
respect,  air  differs  very  materially  from  other  fluids. 

C.  You  told  us,  that  water  was  compressible  in  a 
very  small  degree. 

F.  I  did  so  :  but  the  compression  which  can  be 
effected  with  the  greatest  power  is  so  very  small,  that 
without  the  greatest  attention  and  nicety  in  conductmg 
the  experiments,  it  would  never  have  been  discovered. 
Air,  however,  is  capable  of  being  compressed  into  a 
very  small  space  compared  with  what  it  naturally 
possesses. 

E.  The  experiment  you  made,  by  plungmg  an  ale- 
glass  with  its  mouth  downwards,  clearly  proved  that 
the  air  which  it  contained  was  capable  of  being 
reduced  into  a  smaller  space. 


OF  THE  COMPRESSION  OF  AIR.  271 

F.  This  bended  tube  a  b  c  is  closed  at 
A  and  open  at  c.  It  is  in  the  common 
state,  full  of  air.  I  first  pour  into  it  a 
little  quicksilver,  just  sufficient  to  cover 
the  bottom  a  b ;  now  the  air  in  each  leg 
is  of  the  same  density,  and,  as  that  con- 
tained in  A  B  cannot  escape,  because  the 
lighter  fluid  vv^ill  be  always  uppermost, 
when  I  pour  more  quicksilver  in  at  c,  Fig.  18. 
its  vi^eight  will  condense  the  air  in  the 
leg  A  B,  for  the  air  which  filled  the  whole  length  of 
the  leg  is,  by  the  weight  of  the  quicksilver  in  c  b, 
pressed  into  the  smaller  space  a  x,  which  space  will 
be  diminished  as  the  vv^eight  is  increased :  so  that  by 
increasing  the  length  of  the  column  of  mercury  in  c  b, 
the  air  in  the  other  leg  will  be  more  and  more  con- 
densed. Hence  we  learn  that  the  elastic  spring  of 
air  is  always,  and  under  all  circumstances,  equal  to 
the  force  which  compresses  it. 

C.  How  is  that  proved  ^ 

F.  If  the  spring  with  v/hich  the  an-  endeavours  to 
expand  itself,  when  it  is  compressed,  were  less  than 
the  compressing  force,  it  must  yield  still  farther  to 
that  force  ;  that  is,  if  the  spring  of  the  air  in  a  x  were 
less  than  equal  to  the  weight  of  the  mercury  in  the 
other  leg,  it  would  be  forced  into  a  yet  smaller  space ; 
but  if  the  spring  were  greater  than  the  weight  pressing 
upon  it,  it  would  not  have  yielded  so  much  ;  for  you 
are  well  aware  that  action  and  re-action  are  equal, 
and  act  in  opposite  directions. 

You  can  nov/  easily  understand  why  the  lower 
regions  of  the  atmosphere  are  more  dense  than  those 
which  are  higher. 

£.  Because  they  are  pressed  upon  by  all  the  air 
that  is  above  them,  and  therefore  condensed  into  a 
smaller  space. 

F,  Consequently,  the  air  grows  gradually  thinner, 
till  at  a  considerable  height  it  may  be  conceived  to 
degenerate  to  nothing.  The  different  densities  of  the 
air  may  be  illustrated  by  conceiving  twenty  or  thirty 


272 


PNEUMATICS. 


equal  paclcs  of  wool  placed  one  upon  another;  the 
lowest  will  be  forced  into  a  less  space,  that  is,  its  parts 
will  be  brought  nearer  together,  and  it  will  be  more 
dense,  than  the  next;  and  that  will  be  more  dense 
than  the  thu-d  from  the  bottom,  and  so  on  till  you 
come  to  the  uppermost,  which  sustains  no  other  pres- 
sure than  that  occasioned  by  the  weight  of  the 
incumbent  air. 

Let  us  now  see  the  effects  of  condensed 
air  by  means  of  an  artificial  fountain. 
This  vessel  is  made  of  strong  copper,  and 
about  half  full  of  water.  With  a  syringe 
that  screws  to  the  pipe  b  a  I  force  a  con- 
siderable quantity  of  air  into  the  vessel,  so 
that  it  is  very  much  condensed.  By 
turning  the  stop-cock  b  while  I  take  off 
the  syringe  no  water  can  escape :  and 
instead  of  the  syringe  I  put  on  a  jet,  or 
very  small  tube,  after  which  the  stop- 
cock is  turned,  and  the  pressure  of  the 
condensed  air  forces  the  water  through 
the  tube  to  a  very  great  height. 

C.  Do  you  know  how  high  it  ascends  ? 

-F.  Not  exactly  :  but  as  the  natural  pressure  of  the 
air  will  raise  water  33  feet,  so  if  by  condensation  its 
pressure  be  tripled,  it  will  rise  66  feet. 

E.  Why  tripled  1  Ought  it  not  to  rise  to  this  height 
by  a  double  pressure  ? 

F.  You  forget  that  there  is  the  common  pressure 
always  acting  against,  and  preventing  the  ascent  of 
the  water;  therefore,  besides  a  force  within  to  balance 
that  without,  there  must  be  a  double  pressure. 

C.  You  described  a  syringe  to  be  like  a  common 
water  squirt ;  how  are  you  able,  by  an  instrument  of 
this  kind,  to  force  in  so  great  a  quantity  of  air?  will  it 
not  return  by  the  same  way  it  is  forced  in  1 

F.  'V\\Q  only  difference  between  a  condensing 
syringe  and  a  squirt  is,  that,  in  the  former,  there  is  a 
valve  that  opens  downwards,  by  which  air  may  be 
forced  through  it,  but  the  instant  the  downward  pres- 


Fig.  19. 


OF  THE  COMPRESSION  OF  AIR.  273 
sure  ceases,  the  valve,  by  means  of  a  strong  spriQg, 
shuts  of  itself,  so  that  none  can  return. 

E.  Will  not  air  escape  back,  during  the  time  you 
are  forcing  in  more  of  the  external  air  ? 

F.  That  would  be  the  case  if  the  syringe  pipe  went 
no  low^er  than  that  part  of  the  vessel  which  contams 
the  air,  but  it  reaches  to  a  considerable  depth  in  the 
water,  and  as  it  cannot  find  its  way  back  up  the 
pipe,  it  must  ascend  through  the  water,  and  cause 
that  pressure  upon  it  which  has  been  described. 

C.  To  what  extent  can  air  be  compressed  1 

F.  If  the  apparatus  be  strong  enough,  and  a  suf- 
ficient power  applied,  it  may  be  condensed  several 
thousand  times ;  that  is,  a  vessel  which  will  contain 
a  gallon  of  air  in  its  natural  state  may  be  made  to 
contain  several  thousand  gallons. 

By  means  of  a  fountain  of  this  kind,  young  people, 
like  yourselves,  may  receive  much  entertainment  with 
only  a  few  additional  jets,  which  are  made  to  screw 
on  and  olf.  One  kind  is  so  formed  that  it  will  throw 
up  and  sustain  on  the  stream  a  little  cork  ball,  scat- 
tering the  water  all  around.  Another  is  made  in  the 
form  of  a  globe,  pierced  with  a  great  number  of  holes, 
all  tending  to  the  centre,  exhibiting  a  very  pleasing 
sphere  of  water.  One  is  contrived  to  shew,  in  a  neat 
manner,  the  composition  and  resolution  of  forces  ex- 
plained in  our  Conversations  on  Mechanics.*  Some 
will  form  cascades  ;  and  by  others  you  may,  when  the 
sun  shines  at  a  certain  height  in  the  heavens,  exhibit 
artificial  rainbows.f 

We  will  now  force  in  a  fresh  supply  of  air,  and  try 
some  of  these  jets. 

E.  I  observed  in  the  upright  jets  that  the  height  to 
which  the  water  was  thrown  was  continually  di- 
minishing. 

F.  The  reason  is  this :  that  in  proportion  as  the 

*  See  Conver.  XIIT.  of  Meehanics. 
t  This  phenomenon  we  shall  describe  aiid  explain 
when  we  treat  of  Optics.    (Conver.  XVIII. ") 
N  2 


-74  PNEUMATICS, 
quantity  of  water  in  the  fountain  is  lessened,  the  air 
has  more  room  to  expand,  the  compression  is  di- 
minished, and  consequently  the  pressure  becomes 
less,  till  at  length  it  is  no  greater  within  than  it  is 
without,  and  then  the  fountain  ceases  altogether. 


CONVERSATION  IX. 

MISCELLANEOUS   EXPERIMENTS  ON   THE  AIR-PUMP. 

F.  I  shall,  to-day,  exhibit  a  few  experiments, 
without  any  regard  to  the  particular  subjects  under 
which  they  might  be  arranged. 

In  this  jar  of  water  I  plunge  some  pieces  of  iron, 
zinc,  stone,  &c.,  and  you  will  see  that  when  I  exhaust 
the  external  air,  by  bringing  the  jar  under  the  receiver 
of  the  air-pump,  the  elastic  spring  of  air  contained  in 
the  pores  of  these  solid  substances  will  force  them  out 
in  a  multitude  of  globules,  and  exhibit  a  very  pleasing 
spectacle,  like  the  pearly  dew-drops  on  the  blades  of 
grass  ;  but  when  I  admit  the  air,  they  suddenly  dis- 
appear. 

E.  This  proves  what  you  told  us  a  day  or  two  ago, 
that  substances  in  general  contain  a  great  deal  of 
air. 

F.  Instead  of  bodies  of  this  kind,  I  will  plunge  in 
some  vegetable  substances,  a  piece  or  two  of  the  stem 
of  beet-root,  angelica,  &c.  and  now  observe,  when  I 
have  exhausted  the  receiver,  what  a  quantity  of  air  is 
forced  out  of  the  little  vessels  of  these  plants  by  means 
of  its  elasticity. 

C.  From  this  experiment  we  may  conclude  that  air 
makes  no  small  part  of  all  vegetable  substances. 

F,  To  this  piece  of  cork,  which  of  itself  would 
swim  on  the  surface  of  water,  1  have  tied  some  lead, 


MISCELLANEOUS  EXPERIMENTS.  2?5 
just  enough  to  make  it  sink.  But  by  taking  off  the 
external  pressure,  the  cork  will  bring  the  lead  up  to 
the  surface. 

E.  Is  that  because,  when  the  pressure  is  taken  off, 
the  substance  of  the  cork  expands,  and  becomes  spe- 
cifically lighter  than  it  was  before? 

F,  It  is  :  this  experiment  is  varied  by  using  a  blad- 
der, in  which  is  tied  up  a  very  small  quantity  of  air, 
and  sunk  in  water  :  for  when  the  external  pressure  is 
removed,  the  spring  of  air  within  the  bladder  will 
expand  it,  make  it  specifically  lighter  than  water, 
and  bring  it  to  the  surface. 

The  next  experiment  shews  that  the  ascent  of 
smoke  and  vapours  depends  on  the  air.  I  will  blow 
out  this  candle,  and  put  it  under  the  receiver ;  the 
smoke  now  rises  to  the  top,  but  as  soon  as  the  air  is 
exhausted  to  a  certain  degree,  the  smoke  descends 
like  all  other  heavy  bodies. 

C.  Do  smoke  and  vapours  rise  because  they  are 
lighter  than  the  surrounding  air  ? 

F,  That  is  the  reason  :  sometimes  you  see  smoke 
from  a  chimney  rise  very  perpendicularly  in  a  long 
column  ;  the  air  then  is  very  heavy ;  at  other  times 
you  may  see  it  descend,  which  is  a  proof  that  the 
density  of  the  atmosphere  is  very  much  diminished, 
and  is,  in  fact,  less  than  that  of  the  smoke.  And  at 
all  times  the  smoke  can  ascend  no  higher  than  where 
it  meets  with  air  of  a  density  equal  to  itself,  and  there 
it  will  spread  about  like  a  cloud. 

This  figure  is  usually  called  the  lungs- 
glass  :  a  bladder  is  tied  close  about  the 
little  pipe  ff,  which  is  screwed  into 
the  bottle  a.  I  introduce  it  under  the 
receiver  b,  and  begin  to  exhaust  the 
air  of  the  receiver,  and  that  in  the 
bladder  communicating  with  it,  will 
also  be  withdrawn  ;  the  elastic  force  of 
the  air  in  the  bottle  a  will  now  press 
the  bladder  to  the  shrivelled  state  represented  m  the 
figure  ;  I  will  admit  the  air,  which  expands  '^'^^ 


the 


276  PNEUMATICS. 

bladder  ;  and  thus  by  alternately  exhausting  and  re- 
admitting the  air,  I  shew  the  action  of  the  lungs  in 
breathing.    But  perhaps  the  following  ex- 
periment will  give  a  better  idea  of  the  sub- 
ject.    A  represents  the  lungs,  b  the  wind- 
pipe leading  to  them,  which  is  closely  fixed 
in  the  neck  of  the  bottle,  from  which  the 
air  cannot  escape :  d  is  a  bladder  tied  to 
the  bottom,  and  in  its  distended  state  will, 
with  the  internal  cavity  of  the  bottle,  re- 
present  that  cavity  of  the  body  which  sur-    Fig.  21. 
rounds  the  lungs  at  the  moment  you  have 
taken  in  breath  :  I  force  up  d  (as  in  this     ii  [] 
figure),  and  now  the  bladder  is  shrivelled 
by  the  pressure  of  the  external  air  in  the     [  /'  \  ] 
bottle,  and  represents  the  lungs  just  at  the     j  p] 
moment  of  expiration.  [  jy^/.  | 

E.  Does  Fig.  21.  shew  the  state  of  the  '-^^^ 
lungs  after  I  have  drawn  in  my  breath,  and 

Fig.  22.  when  I  have  thrown  it  out  Fig.  22. 
forcibly  ? 

F.  That  is  what  the  figures  are  intended  to  repre- 
sent, and  they  are  well  adapted  to  shew  the  eleva- 
tion and  compression  of  the  lungs,  although  I  do  not 
mean  to  assert,  that  the  action  of  the  lungs  in  breath- 
ing depends  upon  air  in  the  same  manner  as  that  in 
the  bladder  does  upon  the  air  which  is  contained  in 
the  cavity  of  the  bottle. 

I  have  exactly  balanced  on  this  scale-beam  a  piece 
of  lead  and  a  piece  of  cork  :  in  this  state  I  will  intro- 
duce them  under  the  receiver,  and  exhaust  the  air. 

C.  The  cork  now  seems  to  be  heavier  than  the 
lead. 

F.  In  air  each  body  lost  a  weight  proportional  to 
its  bulk,  but  when  the  air  is  taken  away,  the  weight 
lost  will  be  restored  ;  but  as  the  lead  lost  least,  it  will 
now  retrieve  the  least,  consequently  the  cork  will 
preponderate  with  the  difference  of  the  weights  re- 
stored by  taking  away  the  air. 

Thus  you  see  that,  in  vacuo,  a  pound  of  feathers 


OF  THE  AIR-GUN.  277 
would  be  heavier  than  a  pound  of  lead ;  because,  as 
the  air  is  a  fluid  substance,  it  tends  to  raise  a  body 
immersed  in  it,  and  its  effect  is  proportional  to  the 
bulk  of  the  body. 


CONVERSATION  X. 

OF  THE  AIR-GUN,   AND  SOUND. 

F.  The  air-gun  is  an  instrument,  the  effects  of 
which  depend  on  the  elasticity  and  compression  of 
air. 

E.  Is  it  used  for  the  same  purposes  as  common 
guns  1 

F.  Air-guns  will  answer  all  the  purposes  of  a 
musket  or  fowling-piece  :  bullets  discharged  from 
them  will  kill  animals  at  the  distance  of  50  or  60 
yards.  They  make  no  report,  and  on  account  of  the 
great  mischief  they  are  capable  of  doing,  without 
much  chance  of  discovery,  they  are  deemed  illegal, 
and  are,  or  ought  to  be,  found  no  where  but  among 
the  apparatus  of  the  experimental  philosopher. 

C.  Can  you  shew  us  the  construction  of  an  air- 
gun  ? 

E.  It  was  formerly  a  very  complex  machine,  but 
now  the  construction  of  air-guns  is  very  simple ;  this 
is  one  of  the  most  approved- 


Fig.  23. 


E.  In  appearance  it  is  very  much  like  a  common 
musket,  with  the  addition  of  a  round  ball  c. 

F,  That  ball  is  hollow,  and  co^ntainsthe  condensed 


2T8 


PNEUMATICS. 


air,  into  which  it  is  forced  by  means  of  a  syringe,  and 
then  screwed  to  the  barrel  of  the  gun. 

C,  Is  there  fixed  to  the  bail  a  valve  opening  in- 
w^ards  ? 

J^.  There  is :  and  when  the  leaden  bullet  is  ram- 
med down,  the  trigger  is  pulled  back,  which  forces 
down  the  hook  b  upon  the  pin  connected  with  the 
valve,  and  liberates  a  portion  of  the  condensed  air, 
which  rushing  through  a  hole  in  the  lock  into  the 
barrel,  will  impel  the  bullet  to  a  great  distance. 

E.  Does  net  all  the  air  escape  at  once  1 

F.  No  :  if  the  gun  be  well  made,  the  copper  ball 
will  contain  enough  for  15  or  20  separate  charges  : 
so  that  one  of  these  is  capable  of  doing  much  more 
execution  in  a  given  time  than  a  common  fowling- 
piece. 

C,  Does  not  the  strength  of  the  charges  diminish 
each  time  ? 

-F.  Certainly  ;  because  the  condensation  becomes 
less  upon  the  loss  of  every  portion  of  air  ;  so  that 
after  a  few  discharges  the  ball  will  be  projected  only 
a  short  distance.  To  remedy  this  inconvenience,  you 
might  carry  a  spare  ball  or  two  ready  filled  with  con- 
densed air  in  your  pocket,  to  screw  on  when  the  other 
was  exhausted.  Formerly  this  kind  of  instrument 
was  attached  to  gentlemen's  walking-sticks. 

C.  1  should  like  to  have  one  of  them. 

F.  I  dare  say  you  would  :  but  you  must  not  be 
trusted  with  instruments  capable  of  doing  much  mis- 
chief, till  it  is  quite  certain  that  your  reason  will 
restrain  you  from  actions  that  might  annoy  other 
persons. 

A  still  more  formidable  instrument  is  called  the 
magazine  wind-gun.  In  this  there  is  a  magazine  of 
bullets  as  well  as  another  of  air,  and  when  it  is  pro- 
perly charged  the  bullets  may  be  projected  one  after 
another  as  fast  as  the  gun  can  be  cocked  and  the  pan 
opened.  In  these  the  syringe  is  fixed  to  the  butt  of 
the  gun,  by  which  it  is  easily  charged,  and  may  be 
ke})t  in  that  state  for  a  great  while. 


AHl  THE  MEDIUM  OF  SOUND.  1^79 

E.  Does  air  never  lose  its  elastic  power  ? 

F.  It  would  be  too  much  to  assert  that  it  never 
will :  but  experiments  have  been  tried  upon  different 
portions  of  it,  which  have  been  found  as  elastic  as 
ever  after  the  lapse  of  many  months,  and  even  years. 

C.  What  is  this  bell  for  1 

F.  I  took  it  out  to  shew  you  that  air  is  the  medium 
by  which,  in  general,  sound  is  communicated.  I  will 
place  it  under  the  receiver  of  the  air-pump,  and  ex- 
haust the  air.  Now  observe  the  clapper  of  the  bell 
while  I  shake  the  apparatus. 

E.  I  see  clearly  that  the  clapper  strikes  the  side 
of  the  bell,  but  1  do  not  hear  the  least  noise. 

F,  Turn  the  cock  and  admit  the  air  ;  now  you 
hear  the  sound  plain  enough  and  if  I  use  the  sy- 
ringe and  a  different  kind  of  glass,  so  as  to  condense 
the" air,  the  sound  will  be  very  much  increased.  Dr. 
Desaguliers  says,  that  in  air  that  is  twice  as  dense  as 
comnion  air,  he  could  hear  the  sound  of  a  bell  at 
double  the  distance. 

C.  Is  it  on  account  of  the  different  densities  of  the 
atmosphere,  that  we  hear  St.  Paul's  clock  so  much 
plainer  at  one  time  than  another? 

F.  Undoubtedly  the  different  degrees  of  density  m 
the  atmosphere  will  occasion  some  difference,  but  the 
principal  cause  depends  on  the  quarter  from  which 
the  wind  blows,  for  as  the  direction  of  that  is  towards 
or  opposite  to  our  house,  we  hear  the  clock  better  or 
worse. 

E.  Does  it  not  require  great  strength  to  condense 
air? 

F.  That  depends  much  on  the  size  of  the  pistx)n 
belonging  to  the  syringe  ;  for  the  force  required  in- 
creases in  proportion  to  the  square  of  the  diameter  of 
the  piston. 

Suppose  the  area  of  the  piston  is  one  inch,  and  you 
have  already  forced  so  much  air  into  the  vessel  that 
its  density  is  double  that  of  common  air,  the  resis- 
tance opposed  to  you  will  be  equal  to  15  pounds ; 


280 


PNEUMATICS. 


but  if  you  would  have  it  10  times  as  dense,  the  resis- 
tance will  be  equal  to  150  pounds. 

C  That  would  be  more  than  I  could  manage. 

F.  Well,  then,  you  must  take  a  syringe,  the  area 
of  whose  piston  is  only  half  an  inch  ;  and  then  the 
resistance  would  be  equal  to  only  the  fourth  part  of 
150  pounds,  because  the  square  of  |  is  equal  to  j. 

E.  You  said  that  the  air  was  generally  the  medium 
by  which  sound  is  conveyed  to  our  ears  is  it  not 
always  so  1 

F!  Air  is  always  a  good  conductor  of  sound,  but 
water  is  a  still  better.  Two  stones  being  struck  to- 
gether under  water,  the  sound  may  be  heard  at  a 
greater  distance  by  an  ear  placed  under  water  in  the 
same  river  than  it  can  through  the  air.  In  calm 
weather  a  whisper  may  be  heard  across  the  Thames. 

The  slightest  scratch  of  a  pin  at  one  end  of  a  long 
piece  of  timber,  may  be  heard  by  an  ear  applied  near 
the  other  end,  though  it  could  not  be  heard  at  half 
the  distance  fhrough  the  air. 

The  earth  is  not  a  bad  conductor  of  sound  :  it  is 
said,  that  by  applying  the  ear  to  the  ground,  the 
trampling  of  horses  may  be  heard  much  sooner  than 
it  could  be  through  the  medium  of  the  air.  Kecourse 
has  sometimes  been  had  to  this  mode  of  learning  the 
approach  of  a  hostile  army. 

Take  a  long  stiip  of  flannel,  and  in  the  middle  tie 
a  common  poker,  which  answers  as  well  as  anything, 
leaving  the  ends  at  liberty  :  these  ends  must  be  rolled 
round  the  end  of  the  first  finger  of  each  hand,  and 
then  stopping  the  ears  with  the  ends  of  these  fingers, 
strike  the  poker,  thus  suspended,  against  any  body, 
as  the  edge  of  a  steel  fender  :  the  depth  of  the  tone 
which  the  stroke  will  return  is  amazing ;  that  made 
by  the  largest  church  bell  is  not  to  be  compared  with 
it. — Thus  it  appears  that  flannel  is  an  excellent  con- 
ductor  of  sound. 


OF  SOUND. 


281. 


CONVERSATION  XI. 

OF  SOUND. 

F.  We  shall  devote  this  conversation  to  the  con- 
sideration of  some  curious  circumstances  relating  to 
sound  ;  which,  as  depending  upon  the  air,  will  come 
very  properly  under  Pneumatics. 

C.  You  shewed  us  yesterday  that  the  stroke  made 
by  the  clapper  of  a  bell  was  not  audible,  when  it  was 
under  an  exhausted  receiver;  is  air  the  cause  of 
sound  1 

F.  Certainly  in  many  cases  it  is :  of  this  kind  is 
thunder,  the  most  awful  sound  in  nature. 

E.  Is  thunder  produced  by  the  air  1 

F.  Thunder  is  generally  supposed  to  be  produced 
by  the  concussion  or  striking  together  of  two  bodies 
of  air  ;  for  lightning,  darting  through  the  air,  causes, 
by  its  great  velocity,  a  vacuum,  and  the  separated 
bodies  of  air  rushing  together  produce  the  noise  we 
call  thunder.  The  same  effect,  only  in  miniature,  is 
produced  by  the  inflammation  of  gunpowder. 

C.  Can  the  report  of  a  large  cannon  be  called  a 
miniature  imitation  1  I  remember  being  once^  in  a 
room  at  the  distance  of  but  a  few  paces  of  the  Tower 
guns,  when  they  were  fired,  and  the  noise  was  infi- 
nitely worse  than  any  thunder  that  1  ever  heard. 

F.  This  was  because  you  were  near  to  them  :  gun- 
powder, so  tremendous  as  it  is  in  air,  when  inflamed 
in  a  vacuum  makes  no  more  sound  than  the  bell  in 
like  circumstances. 

Mr.  Cotes  mentions  a  very  curious  experiment, 
which  was  contrived  to  shew  that  sound  cannot  pene- 
trate through  a  vacuum.  A  strong  receiver  filled 
with  common  atmospheric  air,  in  which  a  bell  was 
suspended,  was  screwed  down  to  a  brass  plate  so 
tight  that  no  air  could  escape,  and  this  was  included 
in  a  much  larger  receiver.  When  the  air  between 
the  two  receivers  was  exhausted,  the  sound  of  the  bell 
could  not  be  heard. 


282  PNEUMATICS. 

E.  Could  it  be  heard  before  the  air  was  taken 
away  ? 

F.  Yes  ;  and  also  the  moment  it  was  re-admitted. 
C.  What  is  the  reason  that  some  bodies  sound  so 

much  better  than  others  ?  Bell-metal  is  more  musi- 
cal than  copper  or  brass,  and  these  sound  much  bet- 
ter than  many  other  substances. 

F,  All  sonorous  bodies  are  elastic,  the  parts  of 
which  by  percussion  are  made  to  vibrate  ;  and  as 
long  as  the  vibrations  continue,  corresponding  vibra- 
tions are  communicated  to  the  air,  and  these  produce 
sound.  Musical  chords  and  bells  are  instances  that 
will  illustrate  this. 

E.  The  vibrations  of  the  bell  are  not  visible  ;  and 
musical  chords  will  vibrato  after  their  sound  has 
vanished. 

F.  If  light  particles  of  dust  be  on  the  outside  of  a 
bell  when  it  is  struck,  you  will,  by  their  motion, 
have  no  doubt  but  that  the  particles  of  the  metal 
move  too,  though  not  sufficiently  to  be  visible  to  the 
naked  eye.  And  though  the  motion  of  a  musical 
string  continues  after  the  sound  ceases  to  be  heard, 
yet  it  does  not  follow  that  sound  is  not  still  produced, 
but  only  that  it  is  not  sufficiently  strong  to  produce  a 
sensation  in  the  ear.  You  see  in  a  dark  night  the 
flash  of  a  gun,  but  being  at  a  considerable  distance 
from  it,  you  hear  no  report.  If,  however,  you  knew 
that  the  light  was  occasioned  by  the  inflammation  of 
gunpowder  in  a  musket  or  pistol,  you  would  conclude 
that  it  was  attended  with  sound,  though  it  was  not 
sufficiently  strong  to  reach  the  place  where  you  are. 

C.  Is  it  knov/n  how  far  sound  can  be  heard? 

F,  We  are  assured  upon  good  authority,  that  the 
unassisted  human  voice  has  been  heard  at  the  dis- 
tance of  10  or  12  miles,  namely,  from  New  to  Old 
Gibraltar.  And  in  the  famous  sea-fight  between  the 
English  and  Dutch,  in  1672,  the  sound  of  cannon 
was  heard  at  the  distance  of  200  miles  from  the  place 
of  action. — In  both  these  cases  the  sound  passed  over 
water  ;  and  it  is  well  known  that  sound  may  be 


OF  SOUND. 


283 


always  conveyed  much  farther  along  a  smooth  than 
an  uneven  surface. 

Experiments  have  been  instituted  to  ascertain  how 
much  water,  as  a  conductor  of  sound,  was  better  than 
land  ;  and  a  person  was  heard  to  read  very  distinctly 
at  the  distance  of  140  feet  on  the  Thames,  and  on 
land  he  could  not  be  heard  further  than  76  feet. 

E.  Might  there  not  be  interruptions  in  the  latter 
case  ? 

F,  No  noise  w^hatever  intervened  by  land,  but  on 
the  Thames  there  was  some  occasioned  by  the  flowing 
of  the  water. 

C.  As  we  were  walking  last  summer  towards  Hamp- 
stead,  we  saw  a  party  of  soldiers  firing  at  a  mark  near 
Chalk  Farm,  and  you  desired  Emma  and  me  to  take 
notice,  as  we  approached  the  spot,  how  much  sooner 
the  report  was  heard  after  v/e  saw  the  smoke,  than  it 
was  when  we  first  got  into  the  fields. 

F.  My  intention  was,  that  you  should  know  from 
actual  experiment  that  sound  is  not  conveyed  instan- 
taneously, but  takes  a  certain  time  to  travel  over  a 
given  space.  When  you  stood  close  to  the  place, 
did  you  not  observe  the  smoke  and  hear  the  report  at 
the  same  instant  ? 

£.  Yes,  we  did. 

F.  Then  you  are  satisfied  that  the  light  of  the  flash 
and  the  report  are  always  produced  together.  The 
former  comes  to  the  eye  with  the  velocity  of  light, 
the  latter  reaches  the  ear  with  the  velocity  with  which 
sound  travels  :  if  then  light  travels  faster  than  sound, 
you  will,  at  any  considerable  distance  from  a  gun 
that  is  fired,  see  the  flash  before  you  hear  tlie  report. 
Do  you  know  with  what  velocity  light  travels  ? 

C.  At  the  rate  of  12  millions  of  miles  in  a  minute.* 
F.  With  regard  then  to  several  hundred  yards,  or 
even  a  few  miles,  the  motion  of  light  may  be  consi- 
dered as  instantaneous,  that  is,  there  would  be  no 
assignable  difference  of  time  to  two  observers,  one  of 
whom  should  stand  at  the  breech  of  the  gun,  and  the 
*  See  Conver.  XXVI.  of  Astronomy. 


284 


PNEUMATICS. 


other  at  the  distance  of  six,  or  eight,  or  ten  miles 
from  it. 

-E.  This  I  understand,  because  ten  miles  is  as 
nothing  when  compared  with  12  millions. 

F.  Now  sound  travels  only  at  the  rate  of  about  13 
miles  in  a  minute,  therefore  as  time  is  easily  divisible 
into  seconds,  the  progressive  motion  of  sound  is  rea- 
dily marked  by  means  of  a  stop-watch ;  consequently, 
if  persons  situated,  some  close  to  a  gun  when  it  is 
discharged,  others  at  a  quarter  of  a  mile  from  it,  and 
others  at  half  a  mile,  and  so  on,  they  v/ill  all  see  the 
flash  or  smoke  at  the  same  instant,  but  the  report  will 
reach  them  at  different  times. 

C.  Is  it  certain  that  sounds  of  all  kinds  travel  at 
this  rate  1 

F.  A  great  variety  of  experiments  have  been  made 
on  the  subject,  and  it  seems  now  generally  agreed 
that  sound  travels  with  a  velocity  that  is  equal  to 
1142  feet  in  a  second  of  time. 

E.  Then  with  a  stop-watch  you  could  have  told 
how  far  we  were  from  the  firing  when  we  first  saw  it. 

F,  Most  easily,  for  I  should  have  counted  the 
number  of  seconds  that  elapsed  between  the  flash  and 
the  report,  and  then  have  multiplied  1142  by  the 
number,  and  I  should  have  had  the  exact  distance 
m  feet  between  us  and  the  gun. 

C.  Has  this  knowledge  been  applied  to  any  prac- 
tical purpose  ? 

F.  It  has  frequently  been  used  at  sea,  by  night, 
to  know  the  distance  of  a  ship  that  has  fired  her 
watch-guns.  Suppose  you  were  in  a  vessel,  and  saw 
the  flash  of  a  gun,  and  between  that  and  a  report, 24 
seconds  elapsed,  what  would  be  the  distance  of  one 
vessel  from  another? 

E.  I  should  multiply  1142  by  24,  and  then  bring 
the  product  into  miles,  which  in  this  instance  is  equal 
to  somethmg  more  than  five  miles. 

-F.  The  mischief  occasioned  by  lightning  is  sup- 
posed to  depend  much  on  the  distance  at  which 
the  storm  is  from  the  spot  from  whence  it  is  seen. 


OF  THE  SPEAKING  TRUMPET.  285 
By  counting  the  number  of  seconds  elapsed  be- 
tween the  flash  of  lightning  and  the  clap  of  thunder, 
j'ou  may  ascertain  how  far  distant  you  are  from  the 
storm. 

C.  I  should  like  to  have  a  stop-  watch  to  be  able  to 
calculate  this  for  myself. 

F.  As  it  will,  probably,  be  some  time  before  you 
become  possessed  of  this  expensive  instrument,  I  will 
tell  you  of  something  which  you  have  always  about 
you,  and  which  will  answer  the  purpose. 

E.  What  is  that,  papal 

F.  The  pulse  at  your  wrist,  which  in  healthy  peo- 
ple generally  beats  about  75  times  in  a  minute  in 
the  same  space  of  time  sound  flies  13  miles  :  therefore, 
in  one  pulsation,  sound  passes  over  13  miles  divided  by 
75,  that  is  about  915  feet,  or  the  sixth  part  of  a  mile, 
consequently  in  six  pulsations  it  will  pass  over  a  mile. 

E.  If  I  see  a  flash  of  lightning,  and  between  that 
and  the  thunder  I  count  at  my  wrist  36  or  60  pulsa- 
tions, I  say  the  distance  in  one  case  is  equal  to  6 
miles,  in  the  other  to  10,  because,  if  sound  travel  one- 
sixth  of  a  mile  in  the  interval  between  two  pulsations 

36 

it  will  travel  —  =  6  miles,  during  36  pulsations ;  and 
^  =  10  miles,  during  60  pulsations. 

CONVERSATION  XII. 

OF  THE   SPEAKING  TRUMPET. 

C.  I  have  been  thinking  about  the  nature  of  sound, 
and  am  ready  to  ask  what  it  is ;  I  can  conceive  of 
particles  of  light  issuing  from  the  sun,  or  other  lumi- 
nous bodies,  but  I  know  not  what  sound  is. 

F.  It  would  be  but  of  little  use  to  give  you  a  defini- 
tion of  sound,  but  I  will  endeavour  to  illustrate  the 
subject.  Sound  is  not  a  body  like  light,  but  it  de- 
pends on  the  concussion  or  striking  together  of  other 

*  The  pulse  is  quicker  in  children. 


'286  PNEUMATICS, 
bodies  that  are  elastic,  which  being  put  into  a  tremu- 
lous motion  excite  a  wave  in  the  surrounding  air. 

E.  Is  it  such  a  wave  as  we  see  in  the  pond  when 
it  is  ruffled  by  the  wind? 

F.  Rather  such  a  one  as  is  produced  by  throwing 
a  pebble  into  still  water. 

C.  I  have  often  observed  this  :  the  surface  ot  the 
water  forms  itself  into  circular  waves.        _  ^ 

F.  It  is  probable  that  the  tremulous  motion  of  the 
parts  of  a  sonorous  body  communicate  undulations  in 
the  air  in  a  similar  manner.  Two  obvious  circum- 
stances must  strike  every  observer  with  regard  to  the 
undulations  in  water.  (1.)  The  waves,  the  farther 
they  proceed  from  the  striking  body,  become  less  and 
less,  till,  if  the  water  be  of  a  sufficient  magnitude, 
they  become  invisible,  and  die  away.  The  same 
thing  takes  place  with  regard  to  sound,  the  farther  a 
person  is  from  the  sounding  body,  the  less  plain  it  is 
heard,  till  at  length  the  distance  is  too  great  for  it  to 
be  audible  :  and  (2.)  the  waves  on  the  water  are  not 
propagated  instantaneously,  but  are  formed  one  after 
another  in  a  given  space  of  time.  This,  from  what 
we  have  already  shewn,  appears  to  be  the  manner  in 
which  sound  is  propagated. 

E.  Is  sound  the  effect  which  is  produced  on  the 
ear  by  the  undulations  of  the  air  ? 

F.  It  is  :  and  according  as  these  waves  are  stronger 
or  weaker,  the  impression,  and  consequently  the  sen- 
sation, is  greater  or  less.  If  sound  be  impeded  in  its 
proo-ress  by  a  body  that  has  a  hole  in  it,  the  waves 
pass  through  the  hole,  and  then  diverge  on  the  other 
side  as  from  a  centre.  Upon  this  principle  the  speak- 
2 wo-  trnmipet  is  constructed. 

C.  What  is  that,  sir? 

E.  It  is  a  long  tube,  used  for  the  purpose  ot 
making  the  voice  heard  at  a  considerable  distance  :-— 
the  lenoth  of  the  tube  is  from  6  to  12  or  15  feet,  it  is 
straio-ht''  throughout,  having  at  one  end  a  large  aper- 
ture,°  and  the  other  terminates  in  a  proper  shape  and 
s  ze  to  receive  the  lips  of  the  speaker. 


OF  THF.  SPEAKING  TRUMPET.  237 

E.  Are  these  instruments  much  m  use  1 

F.  It  is  believed  that  they  were  more  used  formerly 
than  now;  they  are  certainly  of  great  antiquity; 
Alexander  the  Great  made  use  of  such  a  contrivance 
to  communicate  his  orders  to  the  army ;  by  means  of 
which  it  is  asserted  he  could  make  himself  perfectly 
understood  at  the  distance  of  10  or  12  miles.  Stentor 
is  celebrated  by  Homer  as  one  who  could  call  louder 
than  fifty  men:  and  from  Stentor  the  speaking- 
trumpet  has  been  called  the  stentorophonic  tube. 

C.  Perhaps  Stentor  was  employed  in  the  army  for 
the  purpose  of  communicating  the  orders  of  the 
general,  and  he  might  make  use  of  a  trumpet  for  the 
purpose,  and  that  is  what  is  meant  by  brazen  lungs. 

F,  This  is  not  an  improbable  conjecture.  Well, 
besides  speaking  trumpets,  there  are  others  contrived 
for  assisting  the  hearing  of  deaf  persons,  which  differ 
but  little  from  the  speaking  trumpet. 

Fig.  24. 

If  A  and  B  represent  two  trumpets,  placed  in  an 
exact  line  at  the  distance  of  40  feet  or  more  from  one 
another,  the  smallest  whisper  at  a  would  be  heard 
distinctly  at  h  ;  so  that  by  a  contrivance  to  conceal 
the  trumpets,  many  of  those  speaking  figures  are 
constructed  which  are  frequently  exhibited  in  the 
metropolis  and  other  large  towns. 

E.  I  see  how  it  may  be  done  ;  there  must  be  two 
sets  of  trumpets,  the  one  connected  with  the  ear  of 
the  image  into  which  the  spectator  whispers,  and 
which  conveys  the  sound  to  a  person  in  another  room, 
who  by  tubes  connected  with  the  mouth  of  the  image 
returns  the  answer. 

C.  How  are  the  lips  set  in  motion  ? 

F.  Very  easily,  by  means  of  a  string  or  wire  pass- 
ing under  the  floor  up  the  body  of  the  image. 


288 


PNEUMATICS- 


CONVERSATION  XIII. 

OF  THE  ECHO. 

F.  Let  US  turn  our  attention  to  another  curious 
subject  relating  to  sound,  and  which  depends  on  the 
air  ;  I  mean  the  echo. 

E.  I  have  often  been  delighted  to  hear  my  own 
words  repeated,  and  I  once  asked  Charles  how  it 
happened,  that  if  I  stood  in  a  particular  spot  in  the 
garden,  and  shouted  loud,  my  words  were  distinctly 
repeated ;  whereas  if  I  moved  a  few  yards  nearer  to 
the  wall  I  had  no  answer.  He  told  me  that  he  knew 
nothing  more  than  this,  that  in  a  part  of  Ovid's  Me- 
tamorphoses, Echo  is  represented  as  having  been  a 
nymph  of  the  woods,  but  that,  pining  away  in  love, 
her  voice  was  all  that  was  left  of  her. 

C.  I  did ;  and  you  shall  hear  a  translation  of  the 
whole  passage  : 

So  wondrous  are  the  effects  of  restless  pain, 
That  nothing  but  her  voice  and  bones  remain,  " 
Nay,  e'en  the  very  bones  at  last  are  g'one, 
And  metamorphosed  to  a  thoughtless  stone. 
Yet  still  the  voice  does  in  the  woods  survive  ; 
The  form's  departed,  but  the  sound's  alive. 

E.  But  these  lines  say  nothing  of  Echo  being  a 
nymph . 

C.  Well,  then,  here  are  others  applied  immediately 
to  Echo: 

A  nymph  she  was,  though  only  now  a  sound, 
Yet  of  her  tongue  no  other  use  was  found 
Than  now  she  has;  which  never  could  be  more 
Than  to  repeat  what  she  had  heard  before. 

F.  I  doubt  this  will  give  your  sister  but  little 
satisfaction  respecting  the  cause  of  the  echo  which  she 
has  often  heard,  and  which  she  may  still  hear,  in  ihe 
garden. 

E,  No,  I  cannot  conceive  why  a  nymph  of  the 


OF  THE  ECHO.  289 
woods  should  take  up  her  residence  in  our  garden  • 
and  the  more  so  as  I  never  saw  her.  ' 

F.  If  she  is  a  mere  sound,  you  cannot  see  her  :  I 
will  endeavour  to  explain  the  subject. — When  you 
throw  a  pebble  into  a  small  pool  of  water,  what  hap- 
pens to  the  waves  when  they  reach  the  margin  1 

C.  They  are  thrown  back  again. 

F.  The  same  happens  with  regard  to  the  undula- 
tions in  the  air,  which  are  the  cause  of  sound.  They 
strike  _  against  any  surface  fitted  for  the  purpose,  as 
the  side  of  a  house,  a  brick  wall,  a  hill,  or  even 
against  trees,  and  are  reflected  or  beat  back  again ; 
this  is  the  cause  of  an  echo. 

E.  I  wonder  then  that  we  do  not  hear  echoes  more 
frequently. 

F .  There  must  be  several  concurring  circumstances 
before  an  echo  can  be  produced.  For  an  echo  to  be 
heard  the  ear  must  be  in  the  line  of  re/lection. 

C.  I  do  not  knov/  what  you  mean  by  the  line  of 
reflection. 

F.  I  cannot  always  avoid  using  terms  that  have 
net  been  previously  explained.  This  is  an  instance. 
I  will,  however,  explain  what  is  meant  by  the  line  of 
incidence,  and  the  line  of  reflection.  When  you 
come  to  Optics  these  subjects  will  be  made  very 
familiar  to  you.    You  can  play  at  marbles  1 

C,  Yes,  and  so  can  Emma. 

F.  It  is  not  a  very  common  amusement  for  girls , 
however,  as  it  happens,  I  shall  find  my  advantage  in 
it,  as  she  will  the  more  readily  enter  into  my  expla- 
nation. 

Suppose  you  were  to  shoot  a  marble  agamst  the 
wainscot;  what  would  happen  ? 

C.  That  depends  on  the  direction  in  which  I  shoot 
it :  if  I  stand  directly  opposite  to  the  wainscot,  the 
marble  will,  if  I  shoot  it  strong  enough,  return  to  my 
hand. 

F.  The  line  which  the  marble  describes  in  going 
to  the  wall  is  called  the  line  of  incidence,  and  that 
which  it  makes  in  returning,  is  the  line  of  reflection. 


290 


PNEUMATICS. 


E.  But  they  are  both  the  same. 

F.  In  tliis  particular  instance  they  are  so  :  but 
suppose  you  shoot  obliquely  or  sideways  against  the 
board,  will  the  marble  return  to  the  hand  1 

C.  Oh  no  :  it  will  fly  off  sideways  in  a  contrary 
direction. 

F.  There  the  line  it  describes  before  the  stroke,  or 
the  line  of  incidence,  is  different  from  that  of  reflec- 
tion,  which  it  makes  after  the  stroke,  I  will  give  you 
another  instance  :  if  you  stand  before  the  looking- 
glass  you  see  yourself,  because  the  rays  of  light  flow 
from  you,  and  are  reflected  back  again  in  the  same 
line.  But  let  Emma  stand  on  one  side  of  the  room, 
and  you  on  the  other  : — you  both  see  the  glass  at  the 
upper  end  of  the  room. 

E.  Yes,  and  I  see  Charles  in  it  too. 

C.  I  see  Emma,  but  I  do  not  see  myself. 

F.  This  happens  just  like  the  marble  which  you 
shot  sideways.  The  rays  flow  from  Emma  obliquely 
on  the  glass,  upon  which  they  strike,  and  fly  off  in  a 
contrary  direction,  and  by  them  you  see  her.  I  will 
apply  this  to  sound.  If 
a  bell  a  be  struck,  and 

the  undulations  of  the  air     /'  V  -  

strike  the  wall  c  in  a  ^  .,/ 
perpendicular  direction, 
they  will  be  reflected  '-^^ 
back  in  the  Scime  line  ; 
and  if  a  person  were  pro- 
perly situated  between  a 
and  r,  as  at  x,  he  would  Fig.  25. 

hear  the  sound  of  the  bell 
by  means  of  the  undulations  as  they  went  to  the  wall, 
and  he  would  hear  it  again  as  they  came  back,  which 
would  be  the  echo  of  the  first  sound. 

E.  I  now  understand  the  distinction  between  the 
direct  sound  and  the  echo. 

E.  If  the  undulations  strike  the  wall  obliquely, 
they  will,  like  the  marble  against  the  wainscot,  or  the 
rays  of  light  against  glass,  fly  off  again  obliquely  on 


OF  THE  ECHO. 


291 


the  other  side,  in  a  reflected  line,  as  c  m  :  now  if  there 
be  a  hill  or  other  obstacle  between  the  bell  and  the 
place  m  where  a  person  happens  to  be  standing,  he 
will  not  hear  the  direct  sound  of  the  bell,  but  only 
the  echo  of  it,  and  to  him  the  sound  will  come  along 
the  line  c  m, 

C.  I  have  heard  of  places  where  the  sound  is  heard 
repeated  several  times. 

F,  This  happens  where  there  are  a  number  of 
walls,  rocks,  &c.  which  reflect  the  sound  from  one  to 
the  other  ;  and  where  a  person  happens  to  stand  in 
such  a  situation  as  to  intercept  all  the  lines  of  reflec- 
tion. These  are  called  tautological  or  babbling 
echoes.  There  can  be  no  echo  unless  the  direct  and 
reflected  sounds  follow  one  another  at  a  sufficient  in- 
terval of  time  ;  for  if  the  latter  arrive  at  the  ear  before 
the  impression  of  the  direct  sound  ceases,  the  sound 
will  not  be  doubled,  but  only  rendered  more  intense. 

E,  Is  there  any  rule  by  which  the  time  may  be 
ascertained  ] 

F,  Yes,  there  is  :  I  will  begin  with  the  most  simple 
case.  If  a  person  stand  at  x,  (Fig.  25.)  in  order  that 
the  echo  may  be  distinct,  the  difference  between  the 
space  ax  and  ac,  added  to  cx^  must  be  at  least  127 
feet. 

C.  The  space  through  which  the  direct  sound 
travels  to  a  person  is  a  x,  and  the  whole  direct  line  to 
the  wall  is  a  c,  besides  which  it  has  to  come  back 
through  cx  to  reach  the  person  again.  AH  this  I 
comprehend :  but  why  do  you  say  127  feet  in  par- 
ticular 1 

F.  It  is  founded  on  this  principle.  By  experience 
it  is  known  that  about  nine  syllables  can  be  articulately 
and  distinctly  pronounced  in  a  second  of  time.  But 
sound  travels  with  the  velocity  of  1142  feet  in  a 
second,  therefore  in  the  ninth  part  of  a  second  it 

passes  over  ^  ,  or  127  feet  nearly,  and  conse- 
quently the  reflected  sound,  which  is  the  echo,  to  be 


292 


PNEUMATICS. 


distinct,  must  travel  over  at  least  127  feet  more  than 
the  direct. 

E.  Ifcd  in  the  figure  represent  the  garden  wall, 
how  far  must  I  be  from  it  to  hear  distinctly  any  word 
I  utter  ?  will  63  or  64  feet  be  sufficient,  so  that  the 
whole  space  which  the  sound  has  to  travel  be  equal  in 
this  case  also  to  127  feet  ? 

F.  It  must  be  something  more  than  this,  because 
the  first  sound  rests  a  certain  time  on  the  ear,  which 
should  vanish  before  the  echo  return,  or  it  will  appear 
a  continuation  of  the  former,  and  not  a  distinct  sound  : 
it  is  generally  supposed  the  distance  must  not  be  less 
than  70  or  72  feet ;  and  this  will  give  the  distinct 
echo  of  one  syllable  only. 

C.  Must  the  distance  be  increased  in  proportion  to 
the  number  of  syllables  that  are  to  be  repeated  ^ 

F,  Certainly  :  and  at  the  distance  of  about  1000 
or  1200  feet,  8  or  10  syllables,  properly  pronounced, 
will  be  distinctly  repeated  by  the  echo. 

But  I  will  finish  this  subject  to-morrow. 


CONVERSATION  XIV. 

OF  THE  ECHO. 

F,  The  following  are  among  the  most  celebrated 
echoes.  At  Rossneath,  near  Glasgow,  there  is  an  echo 
that  repeats  a  tune  played  with  a  trumpet  three  times 
completely  and  distinctly.  In  Gloucestershire,  at 
Thornbury  Castle,  an  echo  repeats  10  or  11  times  dis- 
tinctly. Near  Rome  there  was  one  that  repeated  what 
a  person  said  five  times.  At  Brussels  there  is  an  echo 
that  answers  15  times.  Between  Coblentz  and  Bingen 
an  echo  is  celebrated  as  difl^erent  from  most  others. 
In  common  echoes,  the  repetition  is  not  heard  till 
some  time  after  hearing  the  vvords  spoken  or  notes 
sung ;  in  this  the  person  who  speaks  or  sings  is 
scarcely  heard,  but  the  repetition  very  clearly,  and 
in  surprising  varieties:    the  echo  in  some  cases 


OF  THE  VVHISPEHING  GALLERY.  293 


nppearsto  be  approaching,  in  others  receding:  some- 
times it  is  heard  distinctly,  at  others  scarcely  at  all : 
one  person  hears  only  one  voice,  while  another  hears 
several.  And  to  mention  but  one  more  instance,  in 
Italy,  near  Milan,  the  sound  of  a  pistol  is  returned 
56  times. 

E.  This  is  indeed 

To  fetch  shrill  echoes  from  their  hollow  earth. 

F.  The  ingenious  Mr.  Derham  applied  the  echo  to 
measuring  inaccessible  distances. 

C.  How  did  he  do  this  1 

F,  Standing  on  the  banks  of  the  Thames,  opposite 
Woolwich,  he  observed  that  the  echo  of  a  single 
sound  was  reflected  from  the  houses  in  three  seconds, 
consequently  in  that  time  it  had  travelled  3426  feet, 
the  half  of  which,  or  1713  feet,  was  the  breadth  of  the 
river  in  that  particular  place. 

Did  you  ever  hear  of  the  Whispering-Gallery  in 
the  dome  of  St.  Paul's  Church  ? 

E.  Yes :  and  you  promised  to  take  us  to  see  it 
some  time. 

E.  And  I  will  perform  my  promise.  In  the  mean 
time  it  may  be  proper  to  inform  you,  that  the  circum- 
stance that  attracts  every  person's  attention  is,  that 
the  smallest  whisper  made  against  the  wall  on  one 
side  of  the  gallery  is  distinctly  heard  on  the  other 
side. 

C.  Is  this  effect  produced  on  the  principle  of  the 
echo 

E.  No  :  the  undulations  made  in  the  air  by  the 
voice  are  reflected  both  ways  round  the  wall,  which 
is  made  very  smooth,  so  that  none  may  be  lost,  and 
meet  at  the  opposite  side  ;  consequently  to  the  hearer 
the  sensation  is  the  same  as  if  his  ear  were  close  to 
the  mouth  of  the  speaker. 

E.  Would  the  effect  be  the  same  if  the  two  persons 
were  not  opposite  to  one  another  ? 

E.  In  that  case  the  woids  spoken  would  be  heard 


294 


PNEUMATICS. 


double,  because  one  arc  of  the  circle  being  less  than 
the  other,  the  sound  will  arrive  at  the  ear  sooner 
round  the  shorter  arc  than  round  the  longer  one. 

C.  You  said  the  wall  is  very  smooth  :  is  there  a 
material  difference,  in  the  conveyance  of  sound, 
whether  the  medium  be  rough  or  smooth  ? 

F .  The  difference  is  very  great :  still  water  is, 
perhaps,  the  best  conductor  of  sound :  the  echo 
which  I  mentioned  in  the  neighbourhood  of  Milan, 
depends  much  on  the  water  over  which  the  villa 
stands.  Dr.  Hutton,  in  his  Mathematical  Dictionary, 
gives  the  following  instance  as  a  proof  that  moisture 
has  a  considerable  effect  upon  sound.  A  house  in 
Lambeth-marsh  is  very  damp  during  winter,  when  it 
yields  an  echo,  which  abates  when  it  becomes  dry  in 
summer.  To  increase  the  sound  in  a  theatre,  at 
Home,  a  canal  of  water  was  carried  under  the  floor, 
which  caused  a  great  difference. 

After  water,  stone  is  reckoned  a  good  conductor  of 
sound,  though  the  tone  is  rough  and  disagreeable :  a 
well-made  brick  wall  has  been  known  to  convey  a 
whisper  to  the  distance  of  200  feet  nearly.  Wood  is 
sonorous,  and  produces  the  most  agreeable  tone,  and 
is  therefore  the  most  proper  substance  for  musical  in- 
struments :  of  these  we  shall  say  a  word  or  two  before 
we  quit  the  subject  of  sound. 

-E.  All  wind  instruments,  as  flutes,  trumpets,  &c. 
must  depend  on  the  air :  but  do  stringed  instru- 
ments 

F .  They  all  depend  on  the  vibrations  which  they 
make  in  the  surrounding  air.  I  will  illustrate  what  1 
have  to  say  by  means  of  the  Eolian  harp. 

If  a  cord  eight  or  ten  yards  long  be  stretched  very 
tight  between  two  points,  and  then  struck  with  a 
stick,  the  whole  string  will  not  vibrate,  but  there  will 
be  several  still  places  in  it,  between  which  the  cord 
will  move.  Now  the  air  acts  upon  the  strings  of  the 
harp  in  the  same  manner  as  the  stroke  of  the  stick 
upon  the  long  cord  just  mentioned. 


OF  THE  EOLIAN  HARP.  205 
C  Do  not  tlie  different  notes  upon  a  violin  depend 
upon  the  .Uffercnt  length  of  the  strings,  which  is 
varied  by  the  lingers  of  tlie  musician  I 

F  tLy  do  :  and  the  current  of  air  acts  upon  each 
string,  and  divides  it  into  parts,  as  so  m^^'jy  ^^S'-;"/ 
bridges.  Hence  every  string  m  an  -Lolian  harp 
though  all  are  in  unison,  become  capable  of  severa 
sounds,  from  which  arises  the  wild  and  wonderful 
harmony  of  that  instrument.  ,  ,  ■ 

'fhe  undulations  of  the  air,  caused  by  the  quick 
vibrations  of  a  string,  are  well  illustrated  by  a  sort  of 
mechanical  sympathy  that  exists  among  accordant 
Tounds  If  two  strings  on  different  instruments  are 
uned  n  unison,  and  one  be  struclc,  the  o  her  w,l! 
reply,  Ihough  they  be  several  feet  distant  from  one 
another. 

E.  How  is  this  accounted  tor  !      .     ,  .      n  ,i  ^ 

F.  The  waves  made  by  the  first  string  being  of  the 
same  kind  as  would  be  made  by  the  second  if  st.uck 
tCse  waves  give  a  mechanical  stroke  to  the  second 
ctrino-  and  produce  its  sound. 

C°'lf  all  the  string-son  the  Eolian  harp  are  set  to 
the  same  note,  will  they  all  vibrate  by  striking  only 

""f  They  will :  but  the  fact  is  well  illustrated  in 
this  "method:  bend  little  bits  of      F"-  f 
string,  and  then  strike  one  sufficiently  to  shake  oft 
its  pa;.er,  and  you  will  see  the  others  will  be  shaken 

'T'wm  Sis  happen  if  the  strings  are  not  in 

"xrv  for  yourself,  alter  the  notes  of  all  the 
strin'c^s  but  two,  and  place  the  papers  on  again : 
vibrate  that  string  which  is  in  umson  "'''l;^^""*^/- , 
E.  The  papers  on  those  are  shaken  off;  but  the 

wet  finger  pressed  round  the  edge  of  a  thin 
drinking-glass  will  produce  its  key  ;  if  the  glass  be 
st"uck  so\sto  prodVe  its  pitch,  and  an  uiuson  to 
that  pitch  be  strongly  excited  oa  a  violoncello,  the 


2^^'^  PNEUMATICS. 

glass  will  be  set  in  motion,  and  if  near  the  edge  of 

the  table  will  be  liable  to  be  shaken  off. 


CONVERSATION  XV. 

OF  THE  WINDS. 

F.  Yoa  know,  my  dear  children,  what  the  wind 

C.  You  told  us,  a  few  days  ago,  that  vou  should 
prove  It  was  only  the  air  in  motion. 

F.  I  can  shew  you  in  miniature,  that  air  in  mo- 
tion will  produce  effects  similar  to  those  produced  by 
a  violent  wind. 

I  place  this  little  mill  under  the  receiver  of  the 
air-pump  m  such  a  manner,  that  the  air  when  re- 
entermg  may  catch  the  vanes.  I  will  exhaust  the 
air;— now  observe  what  happens  when  the  stop- 
cock  is  opened.  ^ 
F,  The  vanes  turn  round  with  an  incredible  velo- 
city ;  much  swifter  than  ever  I  saw  the  vanes  of  a 
i-eal  wind-mill.  But  what  puts  the  air  in  motion  so 
as  to  cause  the  actual  wind  ? 

F.  There  are,  probably,  many  conspiring  causes  to 
produce  the  effect.    The  principal  one  seems  to  be 
neat  communicated  by  the  sun. 
C.  Does  heat  produce  wind  ? 
F.  Pleat,  you  know,  expands  all  bodies,  conse- 
quently It  rarefies  the  air,  and  makes  it  lio-hter.  But 
you  have  seen  that  the  lighter  fluids  Ascend,  and 
thereby  leave  a  partial  vacuum,  towards  which  the 
surrounding  heavier  air  presses,  with  a  greater  or  less 
motion,  according  to  the  degree  of  rarefaction  or  of 
heat  which  produces  it.    The  air  of  this  room,  by 
means  of  the  fire,  is  much  warmer  than  that  in  the 
passage. 

E.  Has  that  in  the  passage  a  tendency  into  the 
parlour?  *^ 

F.  Take  thii  lighted  wax  taper,  and  hold  it  at  tlie 
bottom  oi  the  door. 


OP  THE  WINDS. 


29T 


E.  The  wind  blows  the  flame  violently  into  the 
room . 

F,  Hold  it  now  at  the  top  of  the  door. 
C.  The  flame  rushes  outwards  there. 

F.  This  simple  experiment  deserves  your  attention. 
The  heat  of  the  room  rarefies  the  air,  and  the  lighter 
particles  ascending,  a  partial  vacuum  is  made  at  the 
lower  part  of  the  room  ;  to  supply  the  deficiency,  the 
dense  outward  air  rushes  in,  vv'hile  the  lighter  particles, 
as  they  ascend,  produce  a  current  at  the  top  of  the 
door  out  of  the  room.  If  you  hold  the  taper  about  the 
middle  space  between  the  bottom  and  top,  you  will 
find  a  part  in  which  the  flame  is  perfectly  still,  having 
no  tendency  either  inwards  or  outwards. 

The  smoke-jack^  so  common  in  the  chimneys  of 
large  kitchens,  consists  of  a  set  of  vanes,  something 
like  those  of  a  wind-mill,  or  ventilator,  fixed  to  wheel- 
work,  which  are  put  in  motion  by  the  current  of  air 
up  the  chimney,  produced  by  the  heat  of  the  fire ; 
and  of  course  the  force  of  the  jack  depends  on  the 
strength  of  the  fire,  and  not  upon  the  quantity  of 
smoke,  as  the  name  of  the  machine  would  lead  you 
to  suppose. 

E.  Would  you  define  the  wind  as  a  current  of 
air  ? 

F.  That  is  a  very  proper  definition  :  and  its  direc- 
tion is  denominated  from  that  quarter  from  which  it 
blows. 

C.  When  the  wind  blows  from  the  north  or  south, 
do  you  say  it  is  in  the  former  case  a  north  wind,  and 
in  the  latter  a  south  wind  ] 

F.  We  do.  The  winds  are  generally  considered 
as  of  three  kinds,  independently  of  the  names  which 
they  take  from  the  points  of  the  compass  from  which 
they  blow.  These  are  the  constant,  or  those  which 
always  blow  in  the  same  direction  :  the  periodical, 
or  those  which  blow  six  months  in  one  direction,  and 
six  in  a  contrary  direction  :  and  the  variahie,  which 
appear  to  be  subject  to  no  general  rules. 


2m  PNEUMATICS. 

E.  Is  there  any  place  where  the  wind  always 
blows  in  one  direction  only  ? 

F,  This  happens  to  a  very  large  part  of  the 
earth  ;  to  all  that  extensive  tract  that  lies  between  28 
or  30  degrees  north  and  south  of  the  equator. 

C.  What  is  the  cause  of  this  1 

F.  If  you  examine  the  globe  you  will  see  *  that 
the  apparent  course  of  the  sun  is  from  east  to  west, 
and  that  it  is  always  vertical  to  some  part  of  this  tract 
of  our  globe ;  and  since  the  wind  follows  the  sun,  it 
must,  of  necessity,  blow  in  one  direction  constantly 

E.  Arid  is  that  due  east  ? 

F.  It  is  only  so  at  the  equator  :  for  on  the  north  of 
this  line  the  wind  declines  a  little  to  the  north  point 
of  the  compass,  and  this  the  more  so  as  the  place  is 
situated  farther  towards  the  north  ;  on  the  south  side 
the  wind  will  be  southerly. 

C.  The  greater  part  of  this  tract  of  the  globe  is 
water;  and  I  nave  heard  you  say,  that  transparent 
mediunis  do  not  receive  heat  from  the  sun. 

F.  The  greater  part  is  certainly  water  :  but  the 
proportion  of  land  is  not  small :  almost  the  whole 
contment  of  Africa,  a  great  part  of  Arabia,  Persia 
the  East  Indies,  and  China,  besides  the  whole  nearly 
of  New  Holland,  and  numerous  islands  in  the  Indian 
and  Pacific  oceans :  and  in  the  western  hemisphere 
by  far  the  greatest  part  of  South  America,  ]^ew 
Spain,  and  the  West  India  islands,  come  within  the 
limits  of  30  degrees  north  and  south  of  the  equator. 
1  hese  amazingly  large  tracts  of  land  imbibe  the  heat, 
by  which  the  surrounding  air  is  rarefied,  and  thus  the 
wind  becomes  constant ^  or  blows  in  one  direction. 

You  will  also  remember,  that  neither  the  sea'  nor 
the  atmosphere  are  so  perfectly  transparent  as  to 
transmit  all  the  rays  of  the  solar  light ;  many  are 
stopped  m  their  passage,  by  which  both  the  sea  and 

*  It  is  supjiosed  tlie  reader  is  acquainted  M'ith  tlie 
Conversations  on  Astronomy, 


OF  THE  WINDS.  .  290 

air  are  warmed  to  a  considerable  degree.  These 
constant  or  general  winds  are  usually  called  trade^ 
winds. 

E.  In  what  part  of  the  globe  do  the  periodical 
winds  prevail  1 

F,  They  prevail  in  several  parts  of  the  eastern  and 
southern  oceans,  and  evidently  depend  on  the  sun  ; 
for  when  the  apparent  motion  of  that  body  is  north 
of  the  equator,  that  is,  from  the  end  of  March  to 
the  same  period  in  September,  the  wind  sets  m  from 
the  south-west  :  and  the  remainder  of  the  year,  while 
the  sun  is  south  of  the  equator,  the  wind  blows  from 
the  north-east.  These  are  called  the  monsoons,  or 
shifting  trade-winds,  and  are  of  considerable  im- 
portance to  those  who  make  voyages  to  the  East 
Indies. 

C.  Do  these  changes  take  place  suddenly  1 
F,  No;  some  days  before  and  after  the  change 
there  are  calms,  variable  winds,  and  frequently  the 
most  violent  storms. 

On  the  greater  part  of  the  coasts  situated  between 
the  tropics,  the  wind  blows  towards  the  shore  in  the 
day-time,  and  towards  the  sea  by  night.  These  winds 
are  called  sea  and  land  breezes  ;  they  are  affected  by 
mountains,  the  course  of  rivers,  tides,  &c. 

E.  Is  it  the  heat  of  the  sun  by  day  that  rarefies  the 
air  over  the  land,  and  thus  causes  the  wind  ? 

F.  It  is  :  the  following  easy  experiment  will  illus- 
trate the  subject. 

In  the  middle  of  a  large  dish  of  cold  water  put  a 
water-plate  filled  with  hot  water  ;  the  former  repre- 
sents the  ocean,  the  latter  the  land  rarefying  the  air 
over  it.  Hold  a  lighted  candle  over  the  cold  water, 
and  blow  it  out ;  the  smoke,  you  see,  moves  towards 
the  plate.  Reverse  the  experiment  by  filling  the 
outer  vessel  with  warm  water,  and  the  plate  with  cold, 
the  smoke  will  move  from  the  plate  to  the  dish._ 

C.  In  this  country  there  is  no  regularity  m  the 
direction  of  the  winds  ;  sometimes  the  easterly  winds 
prevail  for  several  days  together,  at  other  times  1 


300  Pi^EUMATICS. 

have  noticed  the  wind  blowing  from  all  quarters  of 

the  compass  tvyo  or  three  times  in  the  same  day, 

F.  The  variableness  of  the  wind  in  this  island  de- 
pends probably  on  a  variety  of  causes  ;  for  whatever 
destroys  the  equilibrium  in  the  atmosphere,  produces 
a  greater  or  less  current  of  v/ind  towards  the  place 
where  the  rarefaction  exists. 

It  is  generally  believed  that  the  electric  fluid, 
which  abounds  in  the  air,  is  the  principal  cause  of  the 
variableness  of  the  wind  here.  You  may  often  see 
one  tier  of  clouds  moving  in  a  certain  direction,  and 
another  m  a  contrary  one  ;  that  is,  the  higher  clouds 
will  be  moving  perhaps  north  or  east,  while  the 
weather-cock  stands  directly  south  or  west.  In  cases 
of  this  kind  a  sudden  rarefaction  must  have  taken 
place  m  the  regions  of  one  set  of  these  clouds,  and 
consequently  the  equilibrium  destroyed.  This  phe- 
nomenon is  frequently  found  to  precede  a  thunder- 
storm; from  v^hicb  it  has  been  supposed  that  the 
electric  fluid  is,  in  this  and  such  like  instances,  the 
principal  cause  in  producing  the  wind  :  and  if  in  the 
more  remarkable  appearances  we  are  able  to  trace 
hie  operating  cause,  we  may  naturally  infer  that  those 
which  are  less  so,  but  of  the  same  nature,  depend  on 
a  like  principle. 

E.  Violent  storms  must  be  occasioned  by  sudden 
and  tremendous  concussions  in  nature.  I  remember 
to  have  seen  once  last  year  some  very  large  trees 
torn  up  by  the  wind.  It  is  difficult  to  conceive  how 
so  thin  and  light  a  body  can  produce  such  dire 
effects. 

jP.  The  inconceivable  rapidity  of  lightning  will 
account  for  the  suddenness  of  any  storm;  and°when 
you  are  acquainted  with  what  velocity  a  wind  wiij 
sometimes  move,  you  will  not  be  surprised  at  the 
effects  which  it  is  capable  of  producing. 

C.  Is  there  any  method  of  ascertaining  the  velocity 
of  the  wind  ?  ^ 

F.  Yes  ;  several  machines  have  been  mvented  for 
tiic  purpose.     But  Dr.  Derham,  by  means  of  the 


VELOCITY  OF  THE  WIND.  ^01 
flight  of  small  downy  feathers,  contrived  to  measure 
the  velocity  of  the  great  storm  which  happened  in  the 
year  1705,  and  he  found  the  wind  moved  33  feet  in 
half  a  second,  that  is,  at  the  rate  of  45  miles  per 
hour  :  and  it  has  been  proved,  that  the  force  of  such 
a  wind  is  equal  to  the  perpendicular  force  of  10 
pounds  avoirdupois  Vv^eight  on  every  square  foot. 
Now  if  you  consider  the  surface  which  a  large  tree, 
with  all  its  branches  and  leaves,  presents  to  the  wind, 
you  will  not  be  surprised,  that,  in  great  storms,  some 
of  them  should  be  torn  up  by  the  roots. 

E.  Is  the  velocity  of  45  miles  an  hour  supposed  to 
be  the  greatest  velocity  of  the  wind  ] 

F,  Dr.  Derham  thought  the  greatest  velocity  to 
be  about  60  miles  per  hour.  But  we  have  tables 
calculated  to  shew  the  force  of  the  wind  at  all  velo- 
cities from  1  to  100  miles  per  hour. 

C.  Does  the  force  bear  any  general  proportion  to 
the  velocity  1 

F.  Yes,  it  does :  the  force  increases  as  the  square 
of  the  velocity. 

£.  Do  you  mean,  that  if  on  a  piece  of  board,  ex- 
posed to  a  given  wind,  there  is  a  pressure  equal  to  1 
pound,  and  the  same  board  be  exposed  to  another 
wind  of  double  velocity,  the  pressure  will  be  in  this 
case  4  times  greater  than  it  was  before  1 

F.  That  is  the  rule.  The  following  short  table, 
selected  from  a  larger  one  out  of  Dr.  Hutton's 
Dictionary,  will  fix  the  rule  and  facts  in  your  me^ 
mory 


PNEUMATICS. 
TABLE. 


Velocity  of  the 
iiid,  ill  miles, 
per  hour. 


5 
10 

20 
40 


Perpendicular 
force  on  one 
square  foot  in 
pounds  avoir- 
pois. 


.123 
.492 
1.968 
7.872 
31.488 


Common  appellations  of  the  wind. 


Gentle,  pleasant  wind. 
Brisk  gale. 
Very  brisk. 
Very  high  wind. 
A  hurricane. 


CONVERSATION  XVI. 

OF  THE  STEAM-ENGINE. 

F.  If  you  understand  the  principle  of  the  forcing- 
pump  you  will  easily  comprehend  in  what  manner 
the  steam-engine,  the  most  important  of  all  hydro- 
statical  machines,  acts. 

C.  Why  do  you  call  it  the  most  important  of  all 
machines  ;  is  it  not  a  common  one  ? 

F,  Steam-engines  can  be  used  only  with  advantage 
in  those  cases  where  great  power  is  required.  They 
are  adapted  to  the  raising  of  water  from  ponds  and 
wells  ;  to  the  draining  of  mines;  and  perhaps  with- 
out their  assistance  we  should  not  at  this  moment 
have  the  benefit  of  coal  fires. 

E.  Then  there  cannot  be  two  opinions  entertained 
respecting  their  utility.  I  do  not  know  what  we 
should  do  without  them  in  winter,  or  even  in  summer, 
since  coal  is  the  fuel  chiefly  used  in  dressing  our 
food.  ° 

F.  Our  ancestors  had,  a  century  ago,  excavated 
all  tFie  mines  of  coal  as  deep  as  they  could  be  worked 
without  the  assistance  of  these  sort"  of  engines.  For 
when  the  miners  have  dug  a  certain  depth  below  the 
burface  of  the  earthy  the  vvalcr  pours  in  upon  them  from 


OF  THE  STEAM-ENGINE.  303 
all  skies :  consequently  they  have  no  means  of  going 
on  with  their  work  without  the  assistance  of  a  steam- 
engine,  which  is  erected  by  the  side  of  the  pit,  and 
being  kept  constantly  at  work,  will  keep  it  dry 
enough  for  all  practical  purposes.  ,    .      ,      .  . 

The  steam-engine  was  invented  during  the  reign  ot 
Charles  II.,  though  it  was  not  brought  to  a  degree 
of  perfection  sufficient  for  the  draining  of  mmes  till 
nearly  half  a  century  after  that  period. 

C.  To  whom  is  the  world  indebted  for  the  dis- 

F^^It  is  difficult,  if  not  impossible,  to  ascertain  who 
was  the  inventor.  The  Marquis  of  Worcester  ^de- 
scribed  the  principle  in  a  small  work  entitled  A 
Century  of  Inventions,"  which  was  first  published  in 
the  year  1663. 

E.  Did  the  Marquis  construct  one  of  these  en- 
gines 7  ,  , 

F.  No  ;  the  invention  seems  to  have  been  neg- 
lected for  several  years,  when  Captain  Thomas  ba- 
very,  after  a  variety  of  experiments,  brought  it  o 
some  degree  of  perfection,  by  which  he  was  able  to 
raise  water  in  small  quantities,  to  a  moderate  height. 

C.  Did  he  take  the  invention  from  the  Marquis  ot 
Worcester's  book  ? 

F  Dr.  Desao-uliers,  who,  in  the  middle  ot  the  last 
century,  entered  at  large  into  the  discussion  mam- 
tains  that  Captain  Savery  was  wholly  mdebted  to  the 
Marquis,  and  to  conceal  the  piracy,  he  charges  him 
with  having  purchased  all  the  books  which  contained 
the  discovery,  and  burned  them.  Captain  Savery, 
however,  declared,  that  he  was  led  to  the  discovery 
bvthe  following  accident  Having  drank  a  flask 
of  Florence  wine  at  a  tavern,  and  thrown  the  flask  on 
the  fire,  he  perceived  that  the  few  drops  left  in  it  were 
converted  into  steam,  which  induced  liim  to  snatch 
it  from  the  fire,  and  plunge  its  neck  into  a  bason  ot 
water,  which,  by  the  atmospheric  pressure,  was  driven 
quickly  into  the  bottle."  .      ,    ,  •  . 

E.  This  was  something  like  an  experiment  which 


mi 


P?nEUMATICS. 


I  have  often  seen  at  the  tea-table.  Jf  1  pour  half  a 
cup  of  V,  ater  into  the  saucer,  and  then  hold  a  piece  ot 
lighted  paper  in  the  cup  a  few  seconds,  and  when  the 
cup  is  pretty  warm  plunge  it  with  the  mouth  down- 
wards into  the  saucer,  the  water  almost  instantly  dis- 
appears. 

F,  In  both  cases,  the  principle  is  exactly  the 
same  :  the  heat  of  the  burning  paper  converts  the 
water  that  hung  about  tlie  cup  into  steam,  but  steam 
being  much  lighter  than  air,  expels  the  air  from  the 
cup;  which  being  plunged  into  the  water  the  steam 
is  quickly  condensed,  and  a  partial  vacuum  is  made 
in  the  cup  ;  consequently,  the  pressure  of  the  atmos- 
phere upon  the  water  in  the  saucer  forces  it  into  the 
cup,  just  in  the  same  manner  as  the  water  follows 
the  vacuum  made  in  the  pump. 

C.  Is  steam,  then,  used  for  the  purpose  of  making 
a  vacuum  instead  of  a  piston  ? 

F.  Just  so  :  and  Dr.  Darwin  ascribes  to  Captain 
Savery  the  honour  of  being  the  first  person  who  ap- 
plied it  to  the  purpose  of  raising  water. 

E,  Will  you  describe  the  engine,  that  we  may  see 
how  it  works. 

F ,  I  shall  endeavour  to  give  you  a  general  and 
correct  explanation  of  the  principle  and  mode  of 
acting  of  one  of  Mr.  Watt's  engines,  v/ithout  entering 
into  all  the  minutis  of  the  several  parts. 


OF  THE  STEAM-ENGINE.  S?05 
A  is  a  section  of  the  boiler,  standing  over  a  fire, 
about  half  full  of  water  :  b  is  the  steam-pipe  which 
conveys  the  steam  from  the  boiler  to  the  cylmder  c, 
in  which  the  piston  d,  made  air-tight,  works  up  and 
down,  a  and  c  are  the  steam  valves  through  winch 
the  steam  enters  into  the  cylinder ;  it  is  admitted 
through  a  when  it  is  to  force  the  piston  downwards 
and  through  c  when  it  presses  it  upwards,  b  and  d 
are  the  eduction  valves,  through  which  the  steam 
passes  from  the  cylinder  into  the  condenser  e,  which 
is  a  separate  vessel  placed  in  a  cistern  of  cold  water, 
and  which  has  a  jet  of  cold  water  continually  playing 
up  in  the  inside  of  it.  /  is  the  air-pump,  which  ex- 
tracts  the  air  and  water  from  the  condenser.  It  is 
worked  by  the  great  beam  or  lever  rs,  and  the  water 
taken  from  the  condenser,  and  thrown  into  the  hot 
well  g,  is  pumped  up  again  by  means  of  the  pump  i/, 
and  carried  back  into  the  boiler  by  the  pipe  z  i.  _  k  is 
another  pump,  likewise  worked  by  the  engine  itselj, 
which  supplies  the  cistern,  in  which  the  condenser  is 
fixed,  with  water. 

C.  Are  all  three  pumps,  as  well  as  the  piston, 
worked  by  the  action  of  the  great  beam  1 

F,  They  are  ;  and  vou  see  the  piston-rod  is  fas- 
tened to  the  beam  by^inflexible  bars ;  but,  ttiat  the 
stroke  mio-ht  be  perpendicular,  Mr.  Watt  invented 
the  machinery  called  the  parallel  joint,  the  construc- 
tion of  which  will  be  easily  understood  trom  the 

figure.  1  I,  4.  -J 

E  How  are  the  valves  opened  and  shut  I 
F  Lono-  levers  o  and  p  are  attached  to  them,  which 
are  moved°up  and  down  by  the  piston-rod  of  the  air- 
pump  EF.  In  order  to  communicate  a  rotatory  motion 
to  any  machinery  by  the  motion  of  the  beam,  Mr.  W  att 
makes  use  of  a  large  fly  wheel  x,  on  the  axis  of  which 
is  a  small  concentric  toothed  wheel  n  ;  a  similar 
toothed  wheel  i  is  fastened  to  a  rod  t  coming  from  the 
end  of  the  beam,  so  that  it  cannot  turn  on  its  axis,  but 
must  rise  and  fall  with  the  motion  of  the  great  beam. 
A  bar  of  iron  connects  the  centres  ot  the  two  smaU 


306 


PNEUMATICS. 


toothed  wheels :  when  therefore  the  beam  raises  llie 
Wheel  I,  it  must  move  round  the  circumference  of  the 
wheel  n,  and  with  it  turn  the  fly-wheel  x  ;  which  will 
make  two  revolutions  while  the  wheel  i  goes  round  it 
once.  These  are  called  the  Sun  and  Planet  wheels  ; 
H,  like  the  sun,  turns  only  on  its  axis,  while  i  revolves 
about  it  as  the  planets  revolve  about  the  sun. 

If  to  the  centre  of  the  fly-wheel  any  machinery 
were  fixed,  the  motion  of  the  great  beam  r  s  would 
keep  it  in  constant  work. 

C.  Will  you  describe  the  operation  of  the  engine  ? 
F.  Suppose  the  piston  at  the  top  of  the  cylinder,  as 
it  is  represented  in  the  plate,  and  the  lower  part  of  the 
cylinder  filled  with  steam.    By  means  of  the  pump- 
rod  E  F,  the  steam  valve  a  and  the  eduction  valve  d 
will  be  opened  together,   the  branches  from  them 
being  connected  at  o.    There  being  now  a  communi- 
cation at  d  between  the  cylinder  and  condenser,  the 
steam  is  forced  from  the  former  into  the  latter,  leaving 
the  lower  part  of  the  cylinder  empty,  while  the  steam 
from  the  boiler  entering  by  the  valve  a 
presses  upon  the  piston,  and  forces  it  down.       ,  a 
As  soon  as  the  piston  has  arrived  at  the 
bottom,  the  steam  valve  c  and  the  eduction  fS|j 
valve  b  are  opened,  while  those  at  a  and  d  ny\ 
are  shut ;  the  steam,  therefore,  immediately  IS 
rushes  through  the  eduction  valve  h  into  the 
condenser,  while  the  piston  is  forced  up  ^ 
again  by  the  steam,  which  is  now  admitted 
by  the  valve  c.  Fig. 27, 

CONVERSATION  XVII. 

OF  THE  STEAM-ENGINE. 

C.  I  do  not  understand  how  the  two  sets  of  valves 
act,  which  you  described  yesterday,  as  the  steam  and 
eduction  valves. 

-F.  If  you  look  to  Fig.  27,  there  is  a  different  view 
of  this  part  of  the  machine,  unconnected  with  the  rest  : 
s  is  part  of  the  pipe  which  brings  the  steam  from  the 


OF  THE  STl^AM-ENGINE,  307 
boiler,  a  represents  the  valve,  which,  being  opened, 
admits  the  steam  into  the  upper  part  of  the  cyhnder, 
forcing  down  the  piston. 

E.  Is  not  the  valve  d  opened  at  the  same  time  ' 
F  It  is :  and  then  the  steam  which  was  under  the 
piston  is  forced  through  into  the  condenser  e.  When 
the  piston  arrives  at  the  bottom,  the  other  pan-  ot 
valves  are  opened,  viz.  c  and  b,  through  c  the  steam 
rushes  to  raise  the  piston,  and  through  b  the  steam 
which  pressed  the  piston  down  before,  is  driven  out 
into  the  pipe  r,  leading  to  the  condenser  ;  m  this 
there  is  a  jet  of  cold  water  constantly  playing  up  and 
thereby  the  steam  is  instantly  reduced  into  the  shape 
of  hot  water.  .  -i,        i    r  n 

C.  Then  the  condenser  e  (Fig.  26)  will  soon  be  lull 

of  water.  -,1,1 

F  It  would,  if  it  were  not  connected  by  the  pipe  s 
with  the  pump/.  And  every  time  the  great  beam  r  s 
is  brought  down,  the  plunger  at  the  bottom  of  the  pis- 
ton rod  E  F  descends  to  the  bottom  of  the  pump. 
£.  Is  there  a  valve  in  the  plunger? 
F,  Yes,  which  opens  upwards,  consequently  all  the 
hot  water  which  runs  out  of  the  condenser  into  the 
pump  will  escape  through  the  valve,  and  be  at  the 
top  of  the  plunger,  and  the  valve  not  admittmg  it  to 
return,  it  will,  by  the  ascent  of  the  piston-rod  into  the 
situation  as  is  shewn  in  the  plate,  be  driven  through  n 
into  g,  the  cistern  of  hot  water,  from  which,  owing  to 
a  valve,  it  cannot  return. 

C.  And  I  see  the  same  motion  of  the  great  beam 
puts 'the  pump  y  in  action,  and  brings  over  the  hot- 
water  from  the  cistern  g,  through  the  pipe  i  i  into  the 
little  cistern  v,  which  supplies  the  boiler. 

E.  If  the  pump  h  brings  in,  by  the  same  motion, 
the  water  from  the  well  w,  do  not  the  hot  and  cold 
water  intermix?  ^ 

E.  No:  if  you  look  carefullym  the  figure,  you  will 
•     observe  a  strong  partition  v,  which  separates  the  one 
from  the  other.    Besides,  you  may  perceive  that  the 
hot  water  does  not  stand  at  so  high  a  level  as  the  cold, 


308  PNEUMATICS, 
which  is  a  sufficient  proof  that  they  do  not  communi- 
cate. Indeed,  the  operation  of  the  enoine  woukl  be 
greatly  nijured,  if  not  wholly  stopped,  if  the  hot  water 
commumcated  with  the  cold  ;  as,  in  that  case,  the 
water,  being  at  a  medium  heat,  would  be  too  warm  to 
condense  the  steam  in  e,  and  too  cold  to  be  admitted 
into  the  boiler  ^vithout  checking  the  production  of  the 
steam. 

C.  There  are  some  parts  of  the  apparatus  belongin^^ 
to  the  boiler  which  you  have  not  yet  explained.  What 
is  the  reason  that  the  pipe  q,  which  conveys  the  wat^r 
rom  the  cistern  v  to  the  boiler,  is  turned  up  at  the 
lower  end  1 

F.  If  it  were  not  bent  in  that  manner  the  steam 
that  IS  generated  at  the  bottom  of  the  boiler  would  rise 
into  the  pipe,  and  in  a  great  measure  prevent  the  de- 
scent of  the  water  through  it. 

E,  In  this  position  I  see  clearly  no  steam  can  enter 
the  pipe,  because  steam,  being  much  lighter  than 
\yater,  must  rise  to  the  surface,  and  cannot  possibly 
sink  through  the  bended  part  of  the  tube.  What  does 
m  represent  ? 

F.  It  represents  a  stone  suspended  on  a  wire,  which 
is  shewn  by  the  single  line ;  this  stone  is  nicely  ba- 
lanced by  means  of  a  lever,  to  the  other  end  of  which 
IS  another  wire,  connected  with  the  valve  at  the  top  of 
the  pipe  q,  that  goes  down  from  the  cistern. 

C.  Is  the  stone  so  balanced  as  to  keep  the  valve 
sufficiently  open  to  admit  a  proper  quantity  of  water  ? 
^  F.  It  is  represented  by  the  figure  in  that  situation. 
By  a  principle  in  hydrostatics,*  with  which  you  are 
acquainted,  the  stone  is  partly  supported  by  the  water ; 
if  then  by  increasiog  the  fire,  too  great  an  evaporation 
take  place,  and  the  water  in  the  boiler  sink  below  its 
proper  level,  the  stone  also  must  sink,  which  will 
cause  the  valve  to  open  wider,  and  let  that  from  the 
cistern  come  in  faster.  If,  on  the  other  hand,  the  eva- 
poration be  less  than^  it  ought  to  be,  the  water  will 
have  a  tendency  to  rise  in  the  boiler,  and  with  that 
*  See  Conrer.  XI.  ^ii  Hydrostatics. 


OF  THE  STEAM-ENGINE.  309 
the  stone  must  rise,  and  the  valve  will,  consequently, 
let  the  water  in  with  less  velocity.  By  this  neat  con- 
trivance the  water  in  the  boiler  is  always  kept  at  one 
level. 

E.  What  are  the  pipes  t  and  it  for  ? 

F,  They  are  seldom  used,  but  are  intended  to  shew 
the  exact  height  of  the  water  in  the  boiler.  ^The  one 
at  t  reaches  very  nearly  to  the  surface  of  the  water 
when  it  is  at  the  proper  height :  that  at  u  enters  a  little 
below  the  surface.  If  then  the  Vv^ater  be  at  its  proper 
height,  and  the  cocks  t  and  u  be  opened,  steam  will 
issue  from  the  former,  and  loater  from  the  latter.  But 
if  the  water  be  too  high  it  will  rush  out  at  t  instead  of 
steam  :  if  toa  low,  steam  will  issue  out  of  u  instead  of 

water.  i  •    i  i 

C.  Suppose  things  to  be  as  represented  m  the  plate, 
why  will  the  water  rush  out  of  the  cock  u  if  it  be 
opened '?    It  will  not  rise  above  its  level. 

F.  True  :  but  you  forget  that  there  is  a  constant 
pressure  of  the  steam  on  the  surface  of  the  water  in 
the  boiler  which  tends  to  raise  the  water  in  the  pipe  n. 
This  pressure  would  force  the  water  through  the  pipe, 
as  in  an  artificial  fountain.    Conver.  VIII. 

CONVERSATION  XVIII. 

OF  THE  STEAM-ENGINE,  AND  PAPIn's  DIGESTER. 

C.  We  have  seen  the  structure  of  the  steam-engine 
and  its  mode  of  operation  ;  but  you  have  not  told  us 
the  uses  to  which  it  is  applied. 

F.  The  application  of  this  power  was  at  first  wholly 
devoted  to  the  raising  of  water,  either  from  the  mines, 
which  could  not  be  worked  without  such  aid,  or  to  the 
throwing  it  to  some  immense  reservoir,  for  the  purpose 
of  supplying  with  this  useful  article  places  which  are 
higher  than  the  natural  level  of  the  stream.  But  it  is 
novv  applied  to  various  other  purposes,  such  as  the 
working  of  mills,  the  threshing  of  corn,  and  coining. 
In  making  the  copper  money  now  in  use,  the  ingeni- 
ous Mr.  Boulton  contrived,  by  a  single  operation  of 


310  PNEUMATICS, 
the  steam-engine,  to  roll  the  copper  out  to  a  proper 
thickness,  to  cut  it  into  circular  pieces,  and  to  make 
the  faces  and  the  edge. 

C.  How  is  the  power  of  these  engines  estimated? 

I\  The  power  varies  according  to  the  size.  That 
at  Messrs.  Whitbread's  brewhouse,  to  which  I' have 
had  access,  has  a  cylmder  24  inches  in  diameter,  and 
wdl  perform  the  work  of  24  horses,  working  night  and 
day. 

E,  But  the  horses  cannot  work  incessantly. 

F,  They  will  work  only  eight  hours,  at  the  average, 
out  of  the  24;  therefore,  since  the  engine  is  kept  con- 
tinually at  work,  it  will  perform  the  business  of  72 
horses.  The  coals  consumed  by  this  engine  are  about 
seven  chaldrons  per  week,  or  one  chaldron  in  24 
hours. 

By  the  application  of  different  machinery  to  this 
engine,  it  raises  the  malt  into  the  upper  warehouses, 
and  grinds  it;  it  pumps  the  wort  from  the  under- 
backs  into  the  copper ;  raises  the  wort  into  the  coolers ; 
it  fills  the  barrels  when  the  beer  is  made  ;  and  when 
the  barrels  are  full  and  properly  bunged,  they  are,  by 
the  steam  engine,  driven  into  the  storehouses  in  the 
next  street,  a  distance  of  more  than  a  hundred  yards, 
and  let  down  into  the  cellar. 

E,  I  have  met  with  the  term  explosive  steam  :  why 
is  it  so  called  ? 

F .  From  a  great  variety  of  accidents,  that  have 
happened  through  careless  people,  it  appears  that  the 
expansive  force  of  steam  suddenly  raised  is  much 
stronger  than  even  that  of  gunpowder.  At  the  cannon 
foundry  in  IVIoorfields,  some  years  ago,  hot  metal  was 
poured  into  a  mould  that  accidentally  contained  a 
small  quantity  of  water,  which  was  instantly  converted 
into  steam,  and  caused  an  explosion  that  blew  the 
foundry  to  pieces.  A  similar  accident  happened  at  a 
foundry  in  Newcastle,  which  occurred  from  a  little 
water  having  insinuated  itself  into  a  hollow  brass  ball 
that  was  thrown  into  the  melting  pot. 

C.  These  facts  bring  to  my  mind  a  circumstance 


OF  PAPIN'S  DIGESTER. 


311 


that  I  have  often  heard  you  relate,  as  coming  within 
your  own  knowledge. 

F,  You  do  well  to  remind  me  of  it.  The  fact  is 
worth  recording.  A  gentleman  who  was  carrying  on 
a  long  series  of  experiments,  wished  to  ascertain  the 
strength  of  a  copper  vessel,  and  gave  orders  to  his 
workmen  for  the  purpose.  The  vessel,  however, 
burst  unexpectedly,  and,  in  the  explosion,  it  beat 
down  the  brick  wall  of  the  building  in  which  it  was 
placed,  and  was,  by  the  force  of  the  steam,  carried  15 
or  20  yards  from  it ;  several  of  the  bricks  were  thrown 
70  yards  from  the  spot ;  a  leaden  pipe,  suspended  from 
an  adjoining  building,  was  bent  into  a  right  angle  ; 
and  several  of  the  men  were  so  dreadfully  scalded  or 
bruised,  that  for  many  weeks  they  were  unable  to  stir 
from  their  beds.  A  very  intelligent  person,  who  con- 
ducted the  experiment,  assured  me,  that  he  had  not  the 
smallest  recollection  how  the  accident  happened,  or 
by  what  means  he  got  to  his  bed-room  after  the  explo- 
sion. 

E.  Is  it  by  the  force  of  steam  that  bones  are  dis- 
solved in  Papin's  Digester,  which  you  promised  to  de- 
scribe '?* 

jP.  No ;  that  operation  is 
performed  by  the  great  heat 
produced  in  the  digester.  This 
is  a  representation  of  one  of 
these  machines.  It  is  a  strong 
metal  pot,  at  least  an  inch 
thick  in  every  part ;  the  top  is 
screwed  down,  so  that  no 
steam  can  escape  but  through 
the  valve  v.  Fig.  28. 

C.  What  kind  of  a  valve 
is  it? 

F.  It  is  a  conical  piece  of  brass,  made  to  fit  very 
accurately,  but  easily  moveable  by  the  steam  of  the 
water  v^'hen  it  boils  :  consequently,  in  its  simple  state, 

*  See  Conyer.  IIL  of  Mecliaiiirs. 


S12 


PNEUMATICS. 


the  heat  of  the  water  will  never  be  much  greater  than 
that  of  boiling  water  in  an  open  vessel.  A  steel-yard 
is  therefore  fitted  to  it,  and  by  moving  the  weight  w 
backwards  ot  forwards,  the  steam  will  have  a  lesser  or 
greater  pressure  to  overcome. 

E.  Is  the  heat  increased  by  confining  the  steam  ? 

F.  You  have  seen  that,  in  an  exhausted  receiver, 
water  not  near  so  hot  as  the  boiling  point  will  have 
every  appearance  of  ebullition.  It  is  the  pressure  of 
the  atmosphere  that  causes  the  heat  of  boiling  water 
to  be  greater  in  an  open  vessel,  than  in  one  from 
which  the  air  is  exhausted.  In  a  vessel  exposed  to 
condensed  air,  the  heat  required  to  make  the  water 
boil  would  be  still  greater.  Now,  by  confining  the 
steam,  the  pressure  may  be  increased  to  any  given  de- 
gree. If,  for .  instance,  a  force  equal  to  14  or  15 
pounds  be  put  on  the  valve^  the  pressure  upon  the 
water  will  be  double  that  produced  by  the  atmos- 
phere, and  of  course  the  heat  of  the  water  will  be 
greatly  increased. 

C.  Is  there  no  danger  to  be  apprehended  from  the 
bursting  of  the  vessel  ? 

F.  If  great  care  be  taken  not  to  load  this  valve  too 
much,  the  danger  is  not  very  great.  But  in  experi- 
ments made  to  ascertain  the  strength  of  any  particu- 
lar vessel,  too  great  precautions  cannot  be  taken. 

Under  the  direclion  of  Mr.  Papin,  the  original  in- 
ventor, the  bottom  of  a  digester  was  torn  off  with  a 
wonderful  explosion  :  the  blast  of  the  expanded  water 
blew  all  the  coals  out  of  the  fire-place,  the  remainder 
of  the  vessel  was  hurled  across  the  room,  and  striking 
the  leaf  of  an  oaken  table  an  inch  thick,  broke  it  in 
pieces.  The  least  sign  of  water  could  not  be  dis- 
cerned, and  every  coal  was  extinguished  in  a 
moment. 


OF  THE  BAROMETER.  ^13 
CONVERSATION  XIX. 

OF  THE  BAROMETER. 

F.  As  these  Conversations  are  intended  to  make 
you  familiar  with  all  those  philosophical  instruments 
that  are  in  common  use,  as  well  as  to  explain  the  use 
and  structure  of  those  devoted  to  the  teaching  of 
science,  I  shall  proceed  with  an  account  of  the  ba- 
rometer, which,  with  the  thermometer,  is  to  be  found 
m  almost  every  house.  I  will  shew  you  how  the 
barometer  is  made,  without  any  regard  to  the  frame 
to  which  it  is  attached. 

A  B  is  a  glass  tube,  about  33  or  34 
inches  long,  closed  at  top,  that  is,  in  philo- 
sophical language,  hermetically  sealed; 
D  is  a  cup,  bason,  or  wooden  trough, 
partly  filled  with  quicksilver.    I  fill  the       ^  , 
tube  with  the  quicksilver,  and  then  put      A  k 
my  finger  upon  the  mouth,  so  as  to  pre- 
vent  any  of  it  from  running  out;  I  now 
invert  the  tube,  and  plunge  it  in  the  cup  Fio-.  29. 
B.    You  see  the  mercury  subsides  three  ^ 
or  four  inches ;  and  when  the  tube  is  fixed  to  a  gra- 
duated frame  it  is  called  a  barometer,  or  weather- 
glass, and  you  know  it  is  consulted  by  those  who 
study  and  attend  to  the  changes  of  the  weather. 

E.  Why  does  not  all  the  quicksilver  run  out  of 
the  tube  ? 

..7^ *  ^-  ^'!^  ^^^"^^^  ^^^^"g"  another  question  : 

I  What  is  the  reason  that  water  will  stand  in  an  ex- 
hausted tube,  provided  the  mouth  of  it  be  plunwd 
into  a  vessel  of  the  same  fluid  1 

C.  In  that  case,  the  water  is  kept  in  the  tube  by 
the  pressure  of  the  atmosphere  on  the  surface  of  the 
;  water  into  which  it  is  plunged.  If  you  resort  to  the 
:  same  principle,  in  the  present  instance,  why  does  the 
;  water  stand  33  or  34  feet,  but  the  mercury  only  29  or 
130  inches? 

P 


PNEUMATICS, 
F.  Do  you  not  recollect  that  mercury  is  14  timer, 
heavier  than  water?  therefore,  if  the  pressure  of  the 
atmosphere  will  balance  34  feet  of  water,  it  ought,  on 
the  same  principle,  to  balance  only  a  14th  part  of  that 
height  of  mercury  :  now  divide  34  feet,  or  408  mches, 

^'^E^.'^The  quotient  is  little  more  than  29  inches. 

F*  By  this  method  Torricelli  was  led  to  construct 
the  barometer.  It  had  been  accidentally  discovered 
that  water  could  not  be  raised  more  than  about  34  feet 
in  the  pump.  Torricelli,  on  this,  suspected  that  the 
pressure  of  the  atmosphere  was  the  cause  of  the  ascent 
of  water  in  the  vacuum  made  in  pumps,  and  that  a 
column  of  water  34  feet  high  was  an  exact  counter- 
poise to  a  column  of  air  which  extended  to  the  top  oi 
the  atmosphere.  Experiments  soon  confirmed  the 
truth  of  his  conjectures.  He  then  thought,  tnat  it  .34 
feet  of  water  were  a  counterpoise  to  the  pressure  ot 
the  atmosphere,  a  column  of  mercury  ^  as  much 
shorter  than  34  feet  as  mercury  is  heavier  tnan  water, 
would  likewise  sustain  the  pressure  of  the  atmosphere  : 
he  obtained  a  glass  tube  for  the  purpose,  and  found 
his  reasoning  just. 

C.  Did  he  apply  it  to  the  purpose  of  a  weather- 

^^^F  No :  it  was  not  till  some  time  after  this  that  the 
pressure  of  the  air  was  known  to  vary,  at  different 
times,  in  the  same  place.  As  soon  as  that  was  dis- 
covered, the  application  of  the  Torricellian  tube  o 
predicting  the  changes  of  the  weather  immediately 

succeeded.  .        .   ^         ^       j  r^,. 

C.  A  barometer,  then,  is  an  instrument  used  toi 
measuring  the  weight  or  pressure  of  the  atmosphere. 

F  ThSt  is  the  principal  use  of  the  barometer :  li 
the  air  be  dense,  the  mercury  rises  in  the  tube,  and 
indicates  fair  weather:  if  it  grow the  mercury 
falls,  and  presages  ram,  snow,  &c.  tUp 

The  height  of  the  mercury  in  the  tube  is  called  the 

*  See  the  rules  in  the  last  Conversation  on  Pneumatics. 


OF  THE  BAROMETER.  315 
standard  altitude,  which  in  this  country  fluctuates  be- 
tween 28  and  31  inches,  and  the  difference  between 
the  greatest  and  least  altitudes  is  called  the  scale  oj 
variation. 

E.  Is  the  fluctuation  of  the  mercury  different  m 
other  parts  of  the  world  1 

F.  Within  and  near  the  tropics,  there  is  little  or  no 
variation  in  the  height  of  the  mercury  in  the  barometer 
in  all  weathers ;  this  is  the  case  at  St.  Helena.  At 
Jamaica  the  variation  very  rarely  exceeds  three-tenths 
of  an  inch :  at  Naples  it  is  about  one  inch  :  whereas 
in  England  it  is  nearly  three  inches,  and  at  Peters- 
burgh  it  is  as  much  as  3|  inches. 

C.  The  scale  of  variation  is  the  silvered  plate, 
which  is  divided  into  inches  and  .  tenths  of  an  inch  : 
but  what  do  you  call  the  moveable  index? 

F.  It  IS  called  a  vernier,  from  the  inventor's  name^ 
and  the  use  of  it  is  to  shew  the  fluctuation  of  the  mer- 
cury to  the  hundredth  part  of  an  inch.  The  scale  of 
inches  is  placed  on  the  right  side  of  the  barometer  tube,, 
the  beginning  of  the  scale  being  the  surface  of  the 
mercury  in  the  bason  :  the  vernier  plate^and  index  are 
moveable,  so  that  the  index  may,  at  any  time,  be  set 
to  the  upper  surface  of  the  column  of  mercury. 

E.  I  have  often  seen  you  move  the  index,  but  I  am 
still  at  a  loss  to  conceive  how  you  divide  the  inch  into 
hundredth  parts  by  it. 

F.  The  barometer-plate  is  divided  into  tenths  ;  the 
length  of  the  vernier  is  eleven  tenths,  but  divided  into 
ten  equal  parts. 

C.  Then  each  of  the  ten  parts  is  equal  to  a  tenth 
of  an  inch,  and  a  tenth  part  of  a  tenth. 

F,  True  :  but  the  tenth  part  of  a  tenth  is  equal  ta 
a  hundredth  part,  for  you  remember  that  to  divide  a 
fraction  by  any  number,  is  to  multiply  the  denomina- 
tor of  the  fraction  by  the  number,  thus  —  divided  by 

Suppose  the  index  of  the  vernier  to  comcide  exactly 


316  PNEUMATICS. 

with  one  of  the  divisions  of  the  scale  of  variation,  as 

29.3. 

E.  Then  there  is  no  difficulty  ;  the  height  of  the 
barometer  is  said  to  be  29  inches  and  3  tenths. 

F,  Perhaps,  in  the  course  of  a  few  hours,  you  ob- 
serve that  the  mercury  has  risen  a  very  little  ;  what 
will  you  do  ? 

E.  1  will  raise  the  vernier  even  with  the  mercury. 

F.  And  you  find  the  index  so  much  higher  than 
the  division  3  on  the  scale,  as  to  bring  the  figure  1  on 
the  vernier  even  with  the  second  tenth  on  the  scale. 

E.  Then  the  whole  height  is  29  inches,  2  tenths, 
and  one  of  the  divisions  on  the  vernier,  which  is  equal 
to  a  tenth  and  a  hundredth  ;  that  is,  the  height  of 
the  mercury  is  29  inches,  3  tenths,  and  1  hundredth, 
or  29.31. 

F,  If  figure  2  on  the  vernier  stand  even  with  a 
division  ou  the  scale,  how  should  you  call  the  height 
of  the  mercury? 

E.  Besides  the  number  of  tenths,  I  must  add  2 
hundredths,  because  each  division  of  the  vernier  con- 
tains a  tenth  and  a  hundredth  ;  therefore  I  say  the 
barometer  stands  at  29.32,  that  is,  29  inches,  3  tenths, 
and  2  hundredths. 

F.  Here  is  a  representation  a  b 
of  the  upper  part  of  a  barometer 
tube  ;  the  quicksilver  stands  at 
c  :  from  2  to  a:  is  part  of  the  scale 
of  variation ;  1  to  10  is  the  ver- 
nier, equal  in  length  to  eleven- 
tenths  of  an  inch,  but  divided 
into  10  equal  parts.  In  the  pre- 
sent position  of  the  mercury  the 
figure  1  on  the  vernier  coincides 
exactly  with  29.5  on  the  scale  ; 
and  finding  the  index  stand  be- 
tween the  6th  and  7th  divisions 
on  the  scale,  I  therefore  read  the 
height  29.61 ;  that  is,  29  inches, 
6  tenths,  and  1  hundredth. 


Fig.  30. 


OF  THE  BAROMETER.  317 
C.  I  luidei^tand  the  principle  of  the  barometer, 

but  I  want  a  guide  to  teach  me  how  to  predict  the 

changes  of  the  weather,  which  the  rising  and  faUing 

of  the  mercury  presage. 

F.  I  will  give  you  rules  for  this  purpose  in  a  few 

days.* 

CONVERSATION  XX. 

OF  THE  BAROMETER,  AND  ITS  APPLICATION  TO  THE 
MEASURING  OF  ALTITUDES. 

C.  Is  the  height  of  the  atmosphere  known  ? 

F.  If  the  fluid  air  were  similar  to  water,  that  is, 
every  where  of  the  same  density,  nothing  would  be 
easier  than  to  calculate  its  height.— When  the  ba- 
rometer stands  at  30  inches,  the  specific  gravity  of 
the  atmosphere  is  800  times  less  than  that  of  water  ;t 
but  mercury  is  about  14  times  heavier  than  water, 
consequently  the  specific  gravity  of  mercury  is  to  that 
of  air  as  800  multiplied  by  14  is  to  1  ;  or  mercury  is 
11,200  times  heavier  than  air.  In  the  case  before  us, 
a  column  of  mercury,  30  inches  long,  balances  the 
whole  weight  of  the  atmosphere  ;  therefore,  if  the  air 
was  eauallv  dense  at  all  heights  to  the  top,  its  height 
must  be  11,200  times  30  inches  ;  that  is,  the  column 
of  air  must  be  as  much  longer  than  that  of  the  mer- 
cury, as  the  former  is  lighter  than  the  latter.  Do 
you  understand  me  1 

C.  I  think  I  do  :  11,200  multiplied  by  30  gives 
336,000  inches,  which  are  equal  to  5|  miles  nearly. 

F.  That  would  be  the  height  of  the  atmosphere  if  it 
were  equally  dense  in  all  parts :  but  it  is  found  that 
the  air,  by  its  elastic  quality,  expands  and  contracts, 
and  that  at  3i  miles  above  the  surface  of  the  earth  it 
is  twice  as  rare  as  it  is  at  the  surface  ;  that  at  7  miles 
it  is  4  times  rarer ;  at  10|  miles  it  is  8  times  rarer ; 


*  See  Conversation  XXIV. 
+  See  Conversation  VI. 


31Q  PNEUMATICS. 

at  14  miles  it  is  16  times  rarer ;  and  so  on,  according 

to  the  following 

TABLE. 


o 

r  31 1 

7 

sur- 
th, 

.       2  1 

4 

altitude 

A 

lOi 
1-4 

17| 

ove  the 
the  ear 
e  air  is 

8 

.  16 

.  32 

03 

21' 

-3  o-s  . 

.  64 

24| 

, 

miles 

.  128 

.  256 

Now  if  you  were  disposed  to  carry  on  the  addition 
on  one  side,  and  the  multiplication  on  the  other,  you 
would  find  that,  at  500  miles  above  the  surface  of  the 
earth,  a  single  cubical  inch  of  such  air  as  we  breathe, 
would  be  so  much  rarefied  as  to  fill  a  hollow  sphere, 
equal  in  diameter  to  the  vast  orbit  of  the  planet 
Saturn. 

jE.  Is  it  inferred  from  this  that  the  atmosphere  does 
not  reach  to  any  very  great  height  1 

F.  Certainly  ;  for  you  have  seen  that  a  quart  of 
air  at  the  earth's  surface  weighs  but  about  14  or  15 
grains ;  and  by  carrying  on  the  above  table  a  few 
steps,  you  would  perceive,  that  the  same  quantity, 
only  49  miles  high,  would  weigh  less  than  the  16 
thousandth  part  of  14  grains,  consequently,  at  that 
height,  its  density  must  be  next  to  nothmg.  irom 
experiment  and  calculation  it  is  generally  admitted, 
that  the  atmosphere  at  the  height  of  more  than  45  or 
50  miles  above  the  surface  of  the  earth,  is  not  suf- 
ficiently dense  to  refract  the  rays  of  light ;  conse- 
quently, that  is  generally  denominated  the  height  ot 
the  atmosphere. 

C.  By  comparing  the  state  of  the  atmosphere  at 
the  bottom  and  at  the  top  of  a  mountain,  should  you 
perceive  a  sensible  difference  ? 

F,  We  must  not  trust  to  our  feelings  on  such 
occasions.    The  barometer  will  be  a  sure  guide.  I 


OF  THE  BAROMETER.  3i9 
will  not  trouble  you  with  calculations,  but  mention 
two  or  three  facts,  with  the  conclusions  to  be  drawn 
from  them.  In  ascending  the  Puy  de  Dome,  a  very 
hio-h  mountain  in  France,  the  quicksilver  fell  3i 
inches ;  and  the  height  of  the  mountain  was  found, 
by  measurement,  to  be  3204  feet.  By  a  similar  ex- 
periment upon  Snowden,  in  Wales,  the  quicksilver 
was  found  to  have  fallen  3  inches  8-tenths,  at  the 
heio-ht  of  3720  feet  above  the  surface  of  the  earth.  ^ 

From  these  and  many  other  observations  it  is  in- 
ferred, that  in  ascending  any  lofty  eminence,  the 
mercury  in  the  barometer  will  fall  one-tenth  of  an 
inch  for  every  100  feet  of  perpendicular  ascent. 
This  number  is  not  rigidly  exact,  but  sufficiently  so 
for  common  purposes,  and  it  will  be  easily  remem- 
bered. The  three  following  observations  were  taken 
by  Dr.  Nettleton  near  the  town  of  Halifax  : 

Perpendicii-       Lowest  station      Highest  sta-  . 
hi  altifade  of  the  tion  of  the  Diftereiice. 

in  feet.  Barometer.  Barometer. 

102  29.78  29.66  0.12 

236  29.50  29.23  0.27 

507  30.00  29.45  0.55 

E.  If  I  ascend  a  high  hill,  and,  taking  a  barometer 
with  me,  find  the  mercury  has  fallen  1|  inch,  may  1 
conclude  that  the  hill  is  1500  feet  perpendicular 
height?  1  . 

F.  That  number  will  be  rather  too  large,  but  the 
height  would  be  between  14  and  1500  feet.  Are  you 
aware  how  great  a  pressure  you  are  continually  sus- 
taining? .  ,      T      T  f  1 

E.  No  ;  it  never  came  into  my  head,  i  teel  no 
burden  from  it,  therefore  it  cannot  be  very  great. 

E.  You  sustain  every  moment  a  weight  equal  to 
many  tons,  which  if  it  were  not  balanced  by  the  elas- 
tic force  of  the  air  within  the  body,  would  crush  you 
to  pieces. 

C.  We  might  indeed  have  inferred  that  it  was  con- 


320  PNEUMATICS. 

siderable  from  the  sensations  that  we  felt  when  the  air  j 
was  taken  from  under  our  hands.  But  how,  sir,  do  1 
you  make  out  the  assertion  1 

F.  When  the  barometer  stands  at  29.5  the  pressure 
of  the  air  upon  every  square  inch  is  more  than  equal 
to  14  pounds ;  call  it  14  pounds  for  the  sake  of  even 
numbers,  and  the  surface  of  a  middle-sized  man  is 
14|  feet :  tell  me  now  the  weight  he  sustains. 

C.  I  must  multiply  14  by  the  number  of  square 
inches  in  14|  feet:  now  there  are  144  inches  in  a 
square  foot,  consequently  in  14|  feet  there  are  2088 
square  inches;  therefore,  14  pounds  multiplied  by 
2088  will  give  29,232,  the  number  of  pounds  weight 
which  such  a  person  has  to  bear  up. 

F.  That  is  equal  to  about  13  tons;  now  if  Emma 
reckon  herself  only  half  the  size  of  a  grown  person, 
she  will  sustain  65  tons. 

E.  What  must  the  whole  earth  sustain  1 

F.  This  you  may  calculate  at  your  leisure  ;  I  will 
furnish  you  with  the  rule  : — 

Find  the  diameter  of  the  earth,*  from  which  you 
will  easily  get  the  superficial  measure  in  square 
inches,  and  this  you  must  multiply  by  14,  and  you  get 
the  answer  to  the  question  in  pounds  avoirdupois." 

CONVERSATION  XXI. 

OF  THE  THERMOMETER. 

F.  As  the  barometer  is  intended  to  measure  the 
different  degrees  of  density  of  the  atmosphere,  so  the 
thermometer  is  designed  to  mark  the  changes  in  its 
temperature,  with  regard  to  heat  and  cold. 

E.  Is  there  any  difference  between  the  ther- 
mometer that  is  attached  to  the  barometer,  and  that 
which  hangs  out  of  doors  ? 

F.  No  :  they  are  both  made  by  the  same  person, 
and  are  intended  to  shew  the  same  effects.  But  for 
the  purposes  of  accurate  observation  it  is  usual  to 

*  See  Conversation  VII.  of  Astronomy,  p.  102. 


OF  THE  THERMOMETER.  321 
have  two  instruments,  one  attached  to,  or  near,  the 
barometer,  and  the  other  out  of  doors,  to  which 
neither  the  direct  nor  reflected  rays  of  the  sun  should 
ever  come.  Though  my  thermometers  are  both  of 
the  same  construction,  and  such  as  are  principally 
used  in  this  country,  yet  there  are  others  made  of 
different  materials  and  upon  different  principles. 

C.  Does  not  this  thermometer  consist  of  mercury 
inclosed  in  a  glass  tube  that  is  fixed  to  a  graduated 
frame  1 

F.  That  is  the  construction  of  Fahrenheit's  ther- 
mometer :  but  when  these  instruments  were  first  in- 
vented, about  200  years  ago,  air,  water,  spirits  of  wine, 
and  then  oil,  were  made  use  of,  but  these  have  given 
way  to  quicksilver,  which  is  considered  as  the  best  of 
all  the  fluids,  being  highly  susceptible  of  expansion 
and  contraction,  and  capable  of  exhibiting  a  more 
extensive  scale  of  heat.  Fahrenheit's  thermometer  is 
chiefly  used  in  Great  Britain,  and  Reaumur's  on  the 
continent. 

E.  Is  not  this  the  principle  of  the  thermometer, 
that  the  quicksilver  expands  by  heat,  and  contracts 
by  cold  1 

F.  It  is :  place  your  thumb  on  the  bulb  of  the 
thermometer. 

E.  The  quicksilver  gradually  rises. 

F.  And  it  will  continue  to  rise  till  the  mercury  and 
your  thumb  are  of  equal  heat.  Now  you  have  taken 
away  your  hand,  you  perceive  the  mercury  is  falling 
as  fast  as  it  rose. 

C.  Will  it  come  down  to  the  same  point  at  which 
it  stood  before  Emma  touched  it  ? 

F.  It  will,  unless,  in  this  short  space  of  time,  there 
has  been  any  change  in  the  surrounding  air.  Thus, 
the  thermometer  indicates  the  temperature  of  the  air, 
or,  in  fact,  of  any  body  with  which  it  is  in  contact. 
Just  now  it  was  in  contact  with  your  thumb,  and  it 
rose  in  the  space  of  a  minute  or  two  from  bQ^  to  62^ ; 
had  you  held  it  longer  on  it,  the  mercury  would  have 
risen  still  higher.    It  is  now  falling.    Plunge  it  into 


322 


PNEUMATICS. 


boiling  water,*  and  you  will  find  that  the  mercury 
rises  to  212^.  Afterwards  you  may  place  it  in  ice  in 
its  melting  state,  and  it  will  fall  to  32''. 

E.  Why  are  these  particular  numbers  pitched  on  1 

F.  You  will  not  perhaps  be  satisfied  if  I  tell  you, 
that  the  only  reason  why  212  was  fixed  on  to  mark 
the  heat  of  boiling  water,  and  32  that  to  shew  the 
freezing  point,  was,  because  it  so  pleased  M.  Fahren- 
heit :  this  however  was  the  case. 

C.  I  can  easily  conceive  that  at  the  same  degree  of 
cold  water  will  always  begin  to  freeze  ;  but  surely 
there  are  different  degrees  of  heat  in  boiling  water, 
and  therefore  it  should  seem  strange  to  have  only  one 
number  for  it. 

F,  In  an  open  vessel,  boiling  water  is  always  of 
the  same  heat,  that  is,  provided  the  density  of  the  at- 
mosphere be  the  same  :  and  though  you  increase  your 
fire  in  a  tenfold  proportion,  yet  the  water  will  never 
be  a  single  degree  hotter  ;  for  the  superabundant  heat, 
communicated  to  the  water,  flies  off  in  the  form  of 
steam  or  vapour. 

E.  But  suppose  you  confine  the  steam. 

F.  Before  I  should  attempt  this,  I 
must  be  provided  with  a  strong  vessel, 
or,  as  you  have  seen  under  the  article  of 
the  steam-engine,  it  would  certainly 
burst.  But  in  a  vessel  proper  for  the 
purpose,  water  has  been  made  so  hot  as 
to  melt  solid  lead. 

C.  Will  you  explain  the  construction 
of  the  thermometer  1 

F.  A  B  represents  a  glass  tube,  the  end 
A  is  blown  into  a  bulb,  and  this,  with  a 
part  of  the  tube,  is  filled  with  mercury. 
In  good  thermometers,  the  upper  part 
of  the  tube  approaches  to  a  perfect  vacu- 
um, and  of  course  the  end  b  is  hermeti-    Fig.  31. 

*  This  should  be  done  very  gradually,  by  holding  it 
some  time  in  the  steam,  to  prevent  its  breaking  by  the 
sudden  heat. 


OF  THE  THERMOMETER.  S23 
caliy  sealed.  If  the  tube  be  now  placed  in  pounded 
ice,  the  mercury  will  sink  to  a  certain  point  x,  which 
must  be  marked  on  the  tube,  and  on  the  scale  oppo- 
site to  this  pomt  32  must  be  placed,  which  is  called 
the  freezing  point.  Then  let  it  be  immersed  m 
boiling  water,  the  mercury  will  rise,  and  after  a  few 
minutes  will  become  stationary;  against  that  point 
make  another  mark,  and  write  on  the  scale  212  for 
the  heat  of  boiling  water.  Between  these  points  let 
the  scale  be  divided  into  180  equal  parts. 

E.  Why  130  parts? 

F.  Because  you  begin  from  32,  and  if  you  sub- 
tract that  number  from  212,  the  remainder  will  be 
180.  Also,  below  32,  and  above  212,  set  off  more 
divisions  on  the  scale,  equal  to  the  others.  The  scale 
is  finished  when  you  have  written  against  0  extreme 
cold;  digmnst  32  freezi7ig  point;  against  55  temperate 
heat;  against  76  summer-  heat;  against  98  blood 
heat;  against  112  fever  heat;  against  176  spirits 
boil,  and  against  212  water  boils. 

E.  You  said  the  scale  was  to  be  divided  higher 
than  boiling  water,  but  without  mentioning  the  ex- 
tent. 

F.  The  utmost  extent  of  the  mercurial  thermome- 
ter, both  ways,  are  the  points  at  which  quicksilver 
boils  and  freezes  ;  beyond  these  it  can  be  no  guide  : 
now  the  degree  of  heat  at  which  mercury  boils  is  600, 
and  it  freezes  when  it  is  brought  down  as  low  as  39^ 
or  40«  below  0  ;  consequently,  the  whole  extent  oi 
the  mercurial  thermometer  is  640  degrees.  And 
though  the  cold  is  never  so  intense  in  this  country  as 
to  sink  the  mercury  40«  below  the  freezing  point,  yet 
it  is  in  some  parts  of  Lapland  and  Siberia  ;  and  arti- 
Jicial  cold  may  be  produced  here  equal  to  this. 


324 


PNEUMATICS. 


CONVERSATION  XXII. 

OF  THE  THERMOMETER. 

C.  Is  quicksilver,  when  frozen,  a  solid  metal,  like 
iron  and  other  metals  ? 

F.  It  is  thus  far  similar  to  them,  that  it  is  mallea- 
ble, or  will  bear  hammering.  And  when  quicksilver 
boils,  it  goes  off  in  v3.^om  like  boiling  water,  only 
much  slower.  Hence  it  has  been  inferred,  that  all 
bodies  in  nature  are  capable  of  existing  either  m  a 
solid,  fluid,  or  aeriform  state,  according  to  the  degree 
of  heat  to  which  they  are  exposed. 

E.  I  understand  that  water  may  be  either  solid,  as 
ice,  or  in  its  fluid  natural  state,  or  in  a  state  of  vapour 
or  steam.  ,    n  •  i  c 

F,  I  do  not  wonder  that  you  call  the  fluid  state  ot 
water  its  natural  state,  because  we  are  accustomed, 
in  general,  to  see  it  so,  and  when  it  is  frozen  into  ice, 
there  appears  to  us  in  this  country  a  violence  commit- 
ted upon  nature.  But  if  a  person  from  the  West  or 
East  Indies,  who  had  never  seen  the  effects  of  frost, 
were  to  arrive  in  Great  Britain  during  a  severe  and 
long  continued  one,  such  as  formerly  congealed  the 
surface  of  the  Thames,  unless  he  were  told  to  the 
contrary,  he  would  conclude  that  ice  was  some  mine- 
ral, and  naturally  solid. 

E.  Does  it  never  freeze  in  the  East  or  West 
Indies? 

F.  It  seldom  freezes,  unless  in  very  elevated  situa- 
tions, within  35  degrees  of  the  equator  north  and 
south  :  it  scarcely  ever  hails  in  latitudes  higher  than 
60O.  In  our  own  climate,  and  indeed  in  all  others 
between  35^  and  60^  it  rarely  freezes  till  the  sun's 
meridian  altitude  is  less  than  40  degrees.  The  cold- 
est part  of  the  24  hours  is  generally  about  an  hour 
before  sun-rise,  and  the  warmest  part  of  the  day  is 
usually  between  two  and  four  o'clock  in  the  afternoon 


OF  THE  THERMOMETER.  325 
C.  Are  there  no  degrees  of  heat  higher  than  that 
of  boiling  mercury  ? 

F.  Yes,  a  great  many  :  brass  will  not  melt  till  it 
is  heated  more  than  six  times  hotter  than  boiling  mer- 
cury ;  and  to  melt  cast-iron  requires  a  heat  more  than 
six  times  greater  than  this. 

E.  By  what  kind  of  thermometer  are  these  degrees 
of  heat  measured  1 

F,  The  ingenious  Mr.  Wedgewood  has  invented  a 
thermometer  for  measuring  the  degrees  of  heat  up  to 
32,277^  of  Fahrenheit's  scale. 

C.  Can  you  explain  the  structure  of  his  thermo- 
meter 1 

F,  All  argillaceous  bodies,  or  bodies  made  of  clay, 
are  diminished  in  bulk  by  the  application  of  great 
heat.  The  diminution  commences  in  a  dull  red  heat, 
and  proceeds  regularly  as  the  heat  increases,  till  the 
clay  is  vitrified,  or  is  transformed  into  a  glassy  sub- 
stance. This  is  the  principle  of  Mr.  Wedgewood's 
thermometer. 

E.  Is  vitrification  the  limit  of  this  thermometer  ? 

F.  Certainly :  the  construction  and  application  of 
this  instrument  is  extremely  simple,  and  it  marks  all 
the  different  degrees  of  ignition  from  the  red  heat, 
visible  only  in  the  dark,  to  the  heat  of  an  air  furnace. 
It  consists  of  two  rulers  fixed  on  a  plane,  a  little  far- 
ther asunder  at  one  end  than  at  the  other,  leaving  a 
space  between  them.  Small  pieces  of  alum  and  clay, 
mixed  together,  are  made  just  large  enough  to  enter 
at  the  wide  end  :  they  are  then  heated  in  the  fire 
with  the  body  whose  heat  is  to  be  ascertained.  The 
fire,  according  to  its  heat,  contracts  the  earthy  body, 
so  that,  being  applied  to  the  wide  end  of  the  gauge,  it 
will  slide  on  towards  the  narrow  end,  less  or  more, 
according  to  the  degree  of  heat  to  which  it  has  been 
exposed.* 

*  We  have  in  tlie  former  parts  of  this  work  observed, 
that  all  bodies  are  expanded  by  heat.  The  diminution 
of  the  argillaceous  substances  made  use  of  by  Mr. 


32G  PNEUMATICS, 

Each  degree  of  IMr.  Wedgewood's  thermometer 
answers  to  130  degrees  of  Fahrenheit,  and  he  begins 
his  scale  from  red  heat  fully  visible  in  daylight,  which 
he  finds  to  be  equal  to  1077<'  of  Fahrenheit's  scale  if 
it  could  be  carried  so  high. 

Here  is  a  small  scale  of  heat,  as  it  is  applicable  to 
a  few  bodies  : — 

SCALE  OF  HEAT. 

Fahrenheit. 

Extremity  of  Wedgewood's  «  "\ 

scale  .....  240oL  -  i32>770 
Cast  iron  melts  .  .  at  IGO  13fr2lS77 
Fine  gold  melts  .  .  •  ^2  <  g  S  5237 
Fine  silver  melts  .  .  .28  J  |  (  ^''^^ 
Brass  melts  .  •  •  *  f  I  1  ^^^^ 
Red  heat  visible  by  day       .     0  V,      J  1077 

Mercury  boils  at  GOO 

Lead  melts*  ^^^> 

Bismuth  melts*  460 

Tin  melts*  408 

Milk  boils       .       .       •       •       •       •  -213 

Water  boils  212 

Heat  of  the  human  body      .      .      .     92  to  97 

Water  freezes  -^2 

Milk  freezes  30 

A  mixture  of  snow  and  salt  sinks  the  ther- 
mometer to  ^ 

Mercury  freezes   40^ 

C  You  said  that  Reaumur's  thermometer  was 
chiefly  used  abroad ;  what  is  the  difference  between 
that  and  Fahrenheit's  ?         ,      .  . 

l\  Reaumur  places  the  freezmg  pomt  at  0,  or 

Wedgewood rti)i7e«r5  to  be  an  exception:  but  as  the  con 
tractfon  of  these  does  not  commence  till  they  are  ex- 
posed to  a  red  heat,  it  may  probably  be  accounted  for 
from  the  expulsion  of  the  fluid  particles,  rather  than 
from  any  real  contraction  in  the  solids. 

*  If  these  three  metals  be  mixed  together  by  fusion 
in  the  proportion  of  5,  8,  and  3,  the  mixture  will  melt 
in  a  heat  below  that  of  boiling  water. 


OF  THE  PYROMETER.  327 
zero,  and  each  degree  of  his  thermometer  is  equal  to 
2i,  or  I  degrees  of  Fahrenheit's. 

E.  What  does  he  make  the  heat  of  boiling  water  ? 

F.  Having  fixed  his  freezing  point  at  0,  and 
making  one  of  his  degrees  equal  to  2\  of  Fahrenheit, 
the  heat  of  boiling  water  must  be  SO'*. 

C.  Let  me  see.  The  number  of  degrees  between 
the  freezing  and  boiling  points  on  Fahrenheit's  ther- 
mometer is  180,  which,  divided  by  2|,  or  2.25,  gives 
80  exactly. 

F.  You  have  then  a  rule  by  which  you  may 
always  convert  the  degrees  of  Fahrenheit  into  those  of 
Reaumur:  subtract  32  from  the  given  number, 
and  multiply  by  the  fraction  Tell  me,  Emma, 
what  degree  on  Reaumur's  scale  answers  to  167« 
of  Fahrenheit. 

E,  Taking  32  from  167  there  remains  135,  which, 
multiplied  by  4,  gives  540,  and  this  divided  by  9, 
gives  60.  So  that  60«  of  Reaumur  answers  to  167« 
of  Fahrenheit. 

C.  How  shall  I  reverse  the  operation,  and  find  a 
number  on  Fahrenheit's  scale  that  answers  to  a  given 
one  on  Reaumur's  1 

F.  IMultiply  the  given  number  by  the  improper 
fraction  |,  and  add  32  to  the  product."  Tell  me 
what  number  on  Fahrenheit's  scale  answers  to  40  on 
Reaumur's. 

C.  If  I  multiply  40  by  9,  and  divide  the  product 
by  4,  I  get  90 ;  to  which  if  32  be  added,  the  result 
is  122  ;  which  answers  to  40  on  Reaumur's  scale. 


CONVERSATION  XXIII. 

OF  THE  PYROMETEPw   AND  HYGROMETER, 

F.  To  make  our  description  of  philosophical  instru- 
ments more  perfect,  I  shall  to-day  shew  you  the  con 
struction  and  uses  of  the  pyrometer  and  hygrometer  ; 
and  conclude  to-morrow  with  an  account  of  the  ram- 
gauge  and  some  directions  for  judging  of  the  weather 


328  PNEUMATICS. 

E.  What  do  you  mean  by  a  pyrometer  1 

F.  It  is  a  Greek  word,  and  signifies  a  fire-measurer. 
The  pyrometer  is  a  machine  for  measuring  the  expan- 
sion of  solid  substances,  particularly  metals,  by  heat. 
This  instrument  will  render  the  smallest  expansions 
sensible  to  the  naked  eye. 


C.  Is  all  this  apparatus  necessary  for  the  purpose  ? 

F.  This  as  far  as  I  know  is  one  of  the  most  simple 
pyrometers,  and,  admitting  of  an  easy  explanation, 
1  have  chosen  it  in  preference  to  a  more  complicated 
instrument,  which  might  be  susceptible  of  greater 
nicety. 

To  a  flat  piece  of  mahogany  a  a  are  fixed  three 
studs,  B,  c,  and  d,  and  at  b  there  is  an  adjusting  screw 
p.  H  F  is  an  index,  turning  very  easy  on  the  pivot  f, 
and  L  s  is  another,  turning  on  l,  and  pointing  to  the 
scale  MN.  R  is  part  of  a  watch-spring,  fixed  at  y, 
and  pressing  gently  upon  the  index  l  s.  Here  is  a 
bar  of  iron,  at  the  common  temperature  of  the  sur- 
rounding air  ;  I  lay  it  in  the  studs  c  and  d,  and  ad- 
just the  screw  p  so  that  the  index  ls  may  point  to  0 
on  the  scale. 

C.  The  bar  cannot  expand  without  movmg  the 
mdex  FH,  the  crooked  part  of  which  pressing  upon' 
L  s,  that  also  will  be  moved  if  the  bar  lengthens. 

F.  Try  the  experiment :  friction,  you  know,  pro- 
duces heat ;  take  the  bar  out  of  the  nuts,  rub  it 
briskly,  and  then  replace  it. 

E.  The  index  l  s  has  moved  to  that  part  of  the 


OF  THE  PYROMETER.  329 
scale  which  is  marked  2  :  it  is  now  going  back.  How 
do  you  calculate  the  length  of  the  expansion  '? 

F.  The  bar  pressed  against  the  index  fh  at  f,  and 
that  again  presses  against  ls  at  l,  and  hence  they 
both  act  as  levers. 

C.  And  they  are  levers  of  the  third  kind,  for  in  one 
case  the  fulcrum  is  at  x,  the  power  at  f,  and  the 
point  z  to  be  moved  may  be  considered  as  the  weight : 
—in  the  other,  l  is  the  fulcrum,  the  power  is  applied 
at  r,  and  the  point  s  is  to  be  moved.* 

F.  The  distance  between  the  moving  point  f  and 
H  is  20  times  greater  than  that  between  x  and  f  ;  the 
same  proportion  holds  between  ls  and  Lr ;  from  this 
you  will  get  the  spaces  passed  through  by  the  differ- 
ent points. 

E.  Then  as  much  as  the  iron  bar  expands,  so 
much  will  it  move  the  point  f,  and  of  course  the  point 
z  will  move  20  times  as  much  ;  so  that  if  the  bar 
lengthens  one-tenth  of  an  inch,  the  point  z  would 
move  twenty-tenths,  or  two  inches.  By  the  same 
rule  the  point  s  will  move  through  a  space  20  times 
as  great  as  the  point  r. 

F.  There  are  two  levers,  then,  each  of  which  gam 
power,  or  move  over  spaces,  in  the  proportion  of  20  to 
1  ;  consequently,  when  united,  as  in  the  present  case, 
into  a  compound  lever,  we  multiply  20  into  20,  which 
make  400 ;  and  therefore  if  the  bar  lengthen  one- 
tenth  of  an  inch,  the  point  s  must  move  oyer  400 
times  that  space,  or  40  inches.  But  suppose  it  only 
expands  one  four-hundredth  part  of  an  inch,  how 
much  will  s  move  1 

C.  One  inch. 

F,  But  every  inch  may  be  divided  into  tenths,  and 
consequently,  if  the  bar  lengthen  only  one  four-thou- 
sandth part  of  an  inch,  the  point  s  will  move  through 
the  tenth  of  an  inch,  which  is  very  perceptible.— In 
the  present  case  the  point  s  has  moved  two  inches, 

*  For  an  account  of  the  different  levers,  see  Conver. 
XV.  and  XVI.  of  Mechanics. 


330 


PNEUMATICS. 


therefore  the  expansion  is  equal  to  two  foar-hundredths, 
or  one  two-hundredth  part  of  an  inch. — An  iron  bar, 
three  feet  Jong,  is  about  one  70th  part  of  an  inch 
longer  in  summer  than  in  winter. 

C.  I  see  that,  by  increasing  the  number  of  levers, 
you  might  carry  the  experiment  to  a  much  greater 
degree  of  nicety. 

F.  Well,  let  us  now  proceed  to  the  hygrometer, 
which  is  an  instrument  contrived  for  measuring  the 
different  degrees  of  moisture  in  the  atmosphere. 

E,  I  have  a  weather-house  that  I  bought  at  the 
fair,  which  tells  me  this ;  for  if  the  air  is  very  moist, 
and  thereby  denotes  wet  weather,  the  man  comes  out; 
and  in  fair  weather,  when  the  atmosphere  is  dry,  the 
woman  makes  her  appearance. 

C.  How  is  the  weather-house  constructed  1 

F.  The  two  images  are  placed  on  a  kind  of  lever, 
which  is  sustained  by  catgut ;  and  catgut  is  very  sen- 
sible to  moisture,  twisting  and  shortening  by  mois- 
ture, and  untwisting  and  lengthening  as  it  ^ 
becomes  dry.    On  the  same  principle  is  fj'^ 
constructed  another  hygrometer.    Anisa  \i' 
catgut  string,  suspended  at  a  with  a  little  |[ 
weight  B,  that  carries  an  index  c  round  a        Ij  j-, 
circular  scale  de  on  a  horizontal  board  or  ^^-^A.lx^i 


  -     ^,     «0-:  V-J 

table :  for  as  the  catgut  becomes  moist,  it  ^^^ir^.-^^ 
twists  itself,  and  untwists  when  it  ap-    p-,,.  ^3 
proaches  to  a  dry  state. 

E.  Then  the  degrees  of  moisture  are  shewn  by  the 
index,  which  moves  backwards  and  forwards  by  the 
twisting  and  untwisting  of  the  catgut.  Does  all 
string  twist  with  moisture  1 

F,  Yes.  Take  a  piece  of  common  packthread,  and 
on  it  suspend  a  pound  weight  in  a  vessel  of  water,  and 
you  will  see  how  soon  the  two  strings  are  twisted 
round  one  another. 

C.  I  recollect  that  the  last  time  the  lines  for  dry- 
ing the  linen  were  hung  out  in  the  garden,  that  they 
appeared  to  be  much  looser  in  tiie  evening  than  they 
were  next  morning,  so  that  I  thought  some  person 


0¥  THE  HYGROMETER.  331 
had  been  altering  them.  A  sudden  shower  of  rain 
has  produced  the  same  effect  in  a  striking  manner. 

E.  Sometimes,  when  sudden  damp  weather  has  set 
in,  the  string  of  the  harp  has  snapped  when  no  per- 
son has  been  near  it. 

F.  These  are  the  effects  produced  by  the  moisture 
of  the  air  ;  the  damp  of  night  always  shortens  hair 
and  hempen  lines;  and  owing  to  the  changes  to 
which  the  atmosphere  in  our  climate  is  liable,  the 
harp,  violin,  &c.  that  are  set  to  tune  one  day,  will 
need  some  alteration  before  they  can  be  used  the 
next. 

Here  is  a  sensible  and  very     ^  ^™_======a 

simple  hygrometer  :  it  consists   

of  whipcord,  or  catgut,  fastened  ZZI^l 
at  A,  and  stretched  over  several  IZD^ 
pulleys,  B,  c,  D,  E,  F  ;  at  the 


end  is  a  little  weight  w,  to  which 
is  an  index  pointing  to  a  gradu- 
ated scale.  i'jg-  34. 

C.  Then  according  to  the  de- 
gree of  moisture  in  the  air,  the  string  shortens  or 
lengthens,  and  of  course  the  index  points  higher  or 
lower. 

F.  Another  kind  of  hygrometer 
consists  of  a  piece  of  sponge  e, 
prepared  and  nicely  balanced  on 
the  beam  xy  ;  and  the  fulcrum  z 
lengthened  out  into  an  index 
pointing  to  a  scale  ac.  ^  yIo-  35. 

E.  Does  the  sponge  imbibe  °* 
moisture  sufficiently  to  become  a  good  hygrometer? 

F.  Sponge  of  itself  will  answer  the  purpose,  but  it 
is  made  much  more  sensible  in  the  following  man- 
ner : — 

After  the  sponge  is  well  washed  from  all  impurities 
and  dried  again,  it  should  be  dipped  into  water  or 
vinegar  in  which  sal-ammoniac,  salt  of  tartar,  or 
almost  any  other  salt  has  been  dissolved,  and  then 


332 


PNEUMATICS. 


suffered  to  dry,  when  it  should  be  accurately  ba- 
lanced. 

C.  Do  the  saline  particles  in  damp  weather  im- 
bibe the  moisture,  and  cause  the  sponge  to  prepon- 
derate ? 

F.  They  do.  Instead  of  sponge  a  scale  may  be 
hung  at  E,  in  which  must  be  put  some  kind  of  salt 
that  has  an  attraction  to  the  watery  particles  floating 
in  the  air.  Sulphuric  acid  may  be  substituted  in  the 
place  of  salt,  but  this  is  not  fit  for  your  experiments, 
because  a  little  spilt  over  will  destroy  your  clothes, 
otherwise  it  makes  a  very  sensible  hygrometer. 

E.  I  have  heard  the  cook  complain  of  the  damp 
weather  when  the  salt  becomes  wet  by  it. 

F.  Right :  the  salt-box  in  the  kitchen  is  not  a  bad 
hygrometer ;  and  others  may  be  easily  constructed, 
as  you  extend  your  acquaintance  with  natural  sub- 
stances. 


CONVERSATION  XXV. 


OF  THE  RAIN-GAUGE. 

C.  Does  the  rain-gauge  measure  the  quantity  of 
rain  that  falls  1 

F,  It  shews  the  height  to  which  the  rain  would  rise 
on  the  place  where  it  is  fixed,  if  there  were 
no  evaporation,  and  if  none  of  it  were  im- 
bibed by  the  earth.  One  which  is  made  and 
sold  by  Mr.  Jones,  of  Holborn,  consists  of 
a  funnel  a  communicating  with  a  cylindric 
tube  B.  The  diameter  of  the  funnel  is  ex- 
actly 12  inches,  and  that  of  the  tube  is  4 
inches.  Tell  me,  Emma,  what  propor- 
tion the  area  of  the  former  has  to  that  of 
the  latter. 

E.  I  remember  that  all  plane  surfaces  bear  the  same 
proportion  to  one  another  that  the  squares  of  their 
diameters  have.    Now  the  square  of  12  is  144,  and 


OF  THE  RAIN-GAUGE. 


333 


the  square  of  4  is  16,  therefore  the  proportion  of  the 
area  of  the  funnel  is  to  that  of  the  tube  as  144  to  16. 

F.  But  144  may  be  divided  by  16  without  leaving 
a  remainder. 

C.  Yes,  9  times  16  is  144,  consequently  the  pro- 
portion is  as  9  to  one ;  that  is,  the  area  of  the  fun- 
nel is  9  times  greater  than  that  of  the  tube. 

jP.  If  then  the  water  in  the  tube  be  raised  9  inches, 
the  depth  of  rain  fallen  will,  in  the  area  of  the  funnel, 
which  is  the  true  gauge,  be  only  one  inch. 

E.  Does  the  little  graduated  rule  mark  the  rise? 

F.  Yes,  it  does.  It  is  a  floating  index  divided  into 
inches. 

E.  If  then  the  float  be  raised  1  inch,  is  the  depth 
of  water  reckoned  only  one-ninth  of  an  inch  1 

F,  You  are  right :  and  each  9  inches  in  length 
being  divided  into  100  equal  parts,  the  fall  of  rain  can 
be  readily  estimated  to  the  nine-hundredth  part  of  an 
inch.  Rain-gauges  should  be  varnished  or  well  paint- 
ed, and  as  much  water  should  be  first  poured  in  as  will 
raise  the  float  to  such  a  height,  that  0  or  zero  on  the 
ruler  may  coincide  with  the  edge  of  the  funnel. 

C.  This  is  not  like  your  rain-gauge. 

F.  That  which  I  use,  though  somewhat  more  diffi- 
cult of  explanation,  is  a  much  cheaper  instrument ;  it 
may  without  the  bottle  be  made  for  a  single  shilling. 
It  consists  of  a  tin  funnel ;  the  area  of  the  top  is  ex- 
actly 10  square  inches,  and  the  tube,  about  5  or  7 
inches  long,  passes  through  a  cork  that  is  fixed  in  a 
quart  bottle. 

E.  Is  there  any  particular  proportion  between  the 
area  of  the  funnel  and  that  of  the  bottle  1 

F,  No,  it  is  not  necessary ;  for  in  this  the  quantity  of 
the  rain  is  calculated  by  its  weight  compared  with  the 
area  of  the  funnel,  which  is  known.  For  every  ounce 
of  water  I  allow  .174  parts  of  an  inch  for  the  depth 
of  the  rain  fallen.  Thus  the  last  time  that  I  ex- 
amined the  bottle,  I  found  that  the  water  weighed 
exactly  6  ounces,  and  6  multiplied  by  .174  gives 
1.0443  that  is,  the  rain  fallen  in  the  preceding  month 


334  PNEUMATICS, 
was  equal  to  rather  more  than  1  inch  in  depth.  In 
the  month  of  June  (1801)  the  rain  collected  in  the 
gauge  weighed  11 J  ounces,  which  is  nearly  equal  to 
2  inches  in  depth. 

C.  Pray  explain  the  reason  of  multiplying  the 
number  of  ounces  by  the  decimals  .174. 

F.  Every  imperial  gallon  of  pure  rain  water  con- 
tains 277.3  cubic  inches,  and  weighs  81b.  or  160 
ounces  avoirdupois,  consequently  every  ounce  of 
water  is  equal  to  1.74  cubic  inches  ;  but  the  area  of 
the  funnel  is  10  square  inches,  and  10  multiplied  by 
.174  (the  depth  of  rain  fallen)  is  equal  to  1.74.  , 

You  have  now  a  pretty  full  account  of  all  the  in- 
struments necessary  for  judging  of  the  state  of  the 
weather,  and  for  comparing,  at  different  seasons,  the 
various  changes  as  they  happen. 

E.  Yes  ;  the  barometer  informs  us  how  dense  the  at- 
mosphere is  ;  the  thermometer  its  heat ;  the  hygro- 
meter  what  degree  of  moisture  it  contains  ;  and  by 
the  rain-gauge  how  much  rain  falls  in  a  given  time. 

F.  The  rain-gauge  must  be  fixed  at  some  distance 
from  all  buildings  which  might  in  any  way  shelter  it 
from  particular  driving  winds ;  and  the  height  at 
which  the  surface  of  the  funnel  is  from  the  ground 
must  be  ascertained. 

C.  Does  it  make  any  difference  in  the  quantity  of 
rain  collected  whether  the  gauge  stands  on  the 
ground,  or  some  feet  above  it  ? 

F.  Very  considerable ;  as  that  which  I  have 
described  is  a  cheap  instrument,  one  may  be  placed 
on  the  top  of  the  house,  and  the  other  on  the  garden 
wall,  and  you  will  find  the  difference  much  greater 
than  you  would  imagine.— I  will  now  give  you  some 
rules  for  judging  of,  and  predicting,  the  state  of  the 
weather,  which  are  taken  from  writers  who  have  paid 
the  most  attention  to  these  subjects,  and  which  my 
own  observations  have  verified. 

1.  The  rising  of  the  mercury  presages,  in  general, 
fair  weather  ;  and  its  falling  foul  weather,  as  rain, 
snow,  high  winds,  and  storms.    When  the  surface  of 


OF  JIJDCING  OF  THE  WEATHER.  335 

the  mercury  is  conves:,  or  stands  higher  in  the  middle 
than  at  the  sides,  it  is  a  sign  the  mercury  is  then  in  a 
rising  state  ;  but  if  the  surface  be  concave,  or  hollow 
in  the  middle,  it  is  then  sinking. 

2.  In  very  hot  vv^eather,  the  falling  of  the  mercury 
indicates  thunder. 

3.  In  winter,  the  rising  presages  frost :  and  in 
frosty  weather,  if  the  mercury  falls  three  or  four  divi- 
sions, there  will  be  a  thaw.  But  in  a  continued 
fi'ost,  if  the  meicury  rises,  it  will  certainly  snow. 

4.  When  wet  weather  happens  soon  after  the  de- 
pression of  the  mercury,  expect  but  little  of  it;  on 
the  contrary,  expect  but  little  fair  weather  when  it 
proves  fair  shortly  after  the  mercury  has  risen. 

5.  In  wet  weather,  when  the  mercury  rises  much 
and  high,  and  so  continues  for  two  or  three  days  be- 
fore the  bad  weather  is  entirely  over,  then  a  con- 
tinuance of  fair  weather  may  be  expected. 

6.  In  fair  weather,  when  the  mercury  falls  much 
and  low,  and  thus  continues  for  two  or  three  days  be- 
fore the  rain  comes,  then  a  deal  of  wet  may  be  ex- 
pected,^  and  probably  high  winds. 

7.  The  unsettled  motion  of  the  mercury  denotes 
unsettled  weather. 

8.  The  words  engraved  on  the  scale  are  not  so 
much  to  be  attended  to  as  the  rising  and  falling  of  the 
mercury  :  for  if  it  stand  at  much  rain,  and  then  rises 
to  changeable,  it  denotes  fair  weather,  though  not  to 
continue  so  long  as  if  the  mercury  had  risen  higher. 
If  the  mercury  stands  at  fair,  and  falls  to  changeable, 
bad  weather  may  be  expected. 

9.  In  winter,  spring,  and  autumn,  the  sudden  fall- 
ing of  the  mercury,  and  that  for  a  large  space,  denotes 
high  winds  and  storms  ;  but  in  summer  it  presages 
heavy  showers,  and  often  thunder.  It  always  sinks 
lowest  of  all  for  great  winds,  though  not  accompanied 
with  rain :  but  it  falls  more  for  wind  and  rain  to- 
gether than  for  either  of  them  alone. 

10.  If,  after  rain,  the  wind  change  into  any  part 


336 


PNEUMATICS. 


of  the  north,  with  a  clear  and  dry  sky,  and  the  mer- 
cury rise,  it  is  a  certain  sign  of  fair  weather. 

11.  After  very  great  storms  of  wind,  when  the 
mercury  has  been  low,  it  commonly  rises  again  very 
fast.  In  settled  fair  weather,  except  the  barometer 
sink  much,  expect  but  little  rain.  In  a  v/et  season, 
the  smallest  depressions  must  be  attended  to  j  for 
when  the  air  is  much  inclined  to  showers,  a  little 
sinking  in  the  barometer  denotes  more  rain.  And  in 
such  a  season,  if  it  rise  suddenly  fast  and  high,  fair 
weather  cannot  be  expected  to  last  more  than  a  day 
or  two. 

12.  The  greatest  heights  of  the  mercury  are  found 
upon  easterly  and  north-easterly  winds ;  and  it  may 
often  rain  or  snow,  the  wind  being  in  these  points, 
while  the  barometer  is  in  a  rising  state,  the  effects  of 
the  wind  counteracting.  But  the  mercury  sinks  for 
wind  as  well  as  rain  in  all  other  points  of  the  com- 
pass. 

By  noticing  these,  and  other  rules  which  you  will 
learn  from  experience,  you  will  become  as  well  ac- 
quainted with  the  weather  as  any  persons  can  be  in 
our  variable  climate. 


OPTICS. 


CONVERSATION  L 


INTRODUCTION. 

OF  LIGHT— THE  SMALLNESS  OF  ITS  PARTICLES  THEIR 

VELOCITi  — -THEY  MOVE  ONLY   IN  STRAIGHT  LINES. 

TUTOR  CHARLES  JAMES. 

Charles.  When  we  were  on  the  sea,  you  told  us 
that  you  would  explain  the  reason  why  the  oar, 
which  was  straight  when  it  lay  in  the  boat,  appeared 
crooked  as  soon  as  it  was  put  into  the  water. 

Tutor,  I  did  ;  but  it  requires  some  previous  know- 
ledge before  you  can  comprehend  the  subject.  It 
would  afford  you  but  little  satisfaction  to  bo  told  that 
this  deception  was  caused  by  the  different  degrees  of 
refraction  which  take  place  in  water  and  in  air. 

James,  We  do  not  know  what  you  mean  by  the 
word  refraction. 

T.  It  will  therefore  be  right  to  proceed  with 
caution ;  refraction  is  a  term  frequently  used  in  the 
science  of  optics,  and  this  science  depends  wholly 
on  light. 

J.  What  is  light  1 

T.  It  would,  perhaps,  be  difficult  to  give  a  direct 
answer  to  your  question,  because  we  know  nothing  of 
the  nature  of  light,-  but  by  the  effects  which  it  pro- 
duces. In  reasoning,  however,  on  this  subject,  it  is 
generally  admitted  that  light  consists  of  inconceivably 
small  particles,  which  are  projected,  or  thrown  off, 
from  a  luminous  body  with  great  velocity  in  all 
directions. 

C.  But  how  is  it  known  that  light  is  composed  of 
small  particles  1 

T.  There  is  no  proof  indeed  that  light  is  material, 
Q 


3GS  OPTICS. 

or  composed  of  particles  of  matter,  and  therefore 
I  said  It  was  generally,  not  universally,  admitted  to 
be  so  ;  but  if  it  is  allowed  that  light  is  matter,  then 
the  particles  must  be  small  beyond  all  computation, 
or  in  falling  on  the  eye  they  would  infallibly  blind 
us. 

/.  Does  not  the  light  come  from  the  sun,  in  some 
such  manner  as  it  does  from  a  candle  ? 

T.  This  comparison  will  answer  our  purpose ;  but 
there  appears  to  be  a  great  difference  between  the  two 
bodies  ;  a  candle,  whether  of  wax  or  tallow,  is  soon 
exhausted  ;  but  philosophers  have  never  been  able  to 
observe  that  the  body  of  the  sun  is  diminished  by  the 
light  which  it  incessantly  pours  forth. 

J.  You  say  incessantly  j  but  v/e  see  only  during 
the  hours  of  day. 

C.  That  is  because  the  part  of  the  earth  which  we 
inhabit  is  turned  away  from  the  sun  during  the  night  : 
but  our  midnight  is  mid-day  to  some  other  parts  of  the 
earth. 

T.  Right :  besides,  yon  know  the  sun  is  not  in- 
tended merely  for  the  benefit  of  this  globe,  but  it  is 
the  source  of  light  and  heat  to  six  other  planets,  and 
eighteen  moons  belonging  to  them. 

C.  And  you  have  not  reckoned  the  four  newly  dis- 
covered little  planets,  which  Dr.  Herschel  denomi- 
nates Asteroidsy  but  which  are  known  by  the  name  of 
Ceres  Ferdinandea,  Pallas,  Juno,  and  Vesta. 

T.  Well,  then,  the  sun  to  these  is  the  perpetual 
source  of  light,  heat,  and  motion  ;  and  to  more  distant 
worlds  it  is  a  fixed  star,  and  will  appear  to  some  as 
large  as  Arcturus,  to  others  no  larger  than  a  star  of 
the  sixth  magnitude,  and  to  others  it  must  be  invisible 
unless  the  inhabitants  have  the  assistance  of  glasses,  or 
are  endowed  with  better  eyes  than  ourselves. 

/.  Pray,  sir,  how  swift  do  you  reckon  that  the 
particles  of  light  move  ? 

T.  This  you  will  easily  calculate,  when  you  know, 
that  they  are  only  about  eight  minutes  in  coming  from 
the  sun. 


THE  SUN,  THE  SOURCE  OF  LIGHT.  339 

C.  And  if  you  reckon  that  at  the  distance  of 
ninety-five  millions  of  miles  from  the  earth,  light  pro- 
ceeds at  the  rate  nearly  of  twelve  millions  of  miles  in 
a  minute,  or  at  260,000  miles  in  a  second  of  time. 
But  how  do  you  know  that  it  travels  so  fast? 

T,  It  was  discovered  by  M.  Koemer,  who  observed 
that  the  eclipses  of  Jupiter's  satellites  took  place 
about  sixteen  minutes  later  if  the  earth  was  in  that 
part  of  its  orbit  which  is  farthest  from  Jupiter,  than  if 
it  was  in  the  opposite  point  of  the  heavens. 

C.  I  understand  this  :  the  earth  may  sometimes  be 
in  a  line  between  the  sun  and  Jupiter  ;  and  at  other 
times  the  sun  is  between  the  earth  and  Jupiter ;  and 
therefore,  in  the  latter  case,  the  distance  of  Jupiter 
from  the  earth  is  greater  than  in  the  former,  by  the 
whole  length  of  the  diameter  of  its  orbit. 

T,  In  this  situation  the  eclipse  of  any  of  the  satel- 
lites is,  by  calculation,  sixteen  minutes  later  than  it 
would  be  if  the  earth  were  between  Jupiter  and  the 
sun  :  that  is,  the  light  flowing  from  Jupiter's  satellites 
is  about  sixteen  minutes  in  travelling  the  diameter  of 
the  earth's  orbit,  or  190  millions  of  miles- 

J.  It  would  be  curious  to  calculate  how  much 
faster  light  travels  than  a  cannon  ball. 

T.  Suppose  a  cannon  ball  to  travel  at  the  rate  of 
twelve  miles  a  minute,  and  light  to  move  a  million 
of  times  faster  than  that ;  yet  Dr.  Akenside  conjec- 
tures that  there  may  be  stars  so  distant  from  us  that 
the  light  proceeding  from  them  has  not  yet  reached 
the  earth :  but  Huygens,  an  eminent  astronomer, 
threw  out  the  idea  before  Akenside  was  born. 

/.  And  you  say  the  particles  of  light  move  in  all 
directions. 

T.  Here  is  a  sheet  of  thick  brown  paper — I  make 
only  a  small  pin-hole  in  it,  and  then,  through  that 
hole,  I  can  see  the  same  objects,  such  as  the  sky,  trees, 
houses,  &c.  as  I  could  if  the  paper  were  not  there. 

C.  Do  we  only  see  objects  by  means  of  the  rays 
of  light  which  flow  from  them  ? 


340 


OPTICS. 


r.  In  no  other  way  :  and  therefore  the  liglit  that 
comes  from  the  landscape  which  I  view  by  looking 
through  the  small  hole  in  the  paper,  must  come  in  all 
directions  at  the  same  time. — Take  another  instance  : 
if  a  candle  be  placed  on  an  eminence  in  a  dark  night, 
it  may  be  seen  all  round  for  the  space  of  half  a  mile  : 
in  other  words,  there  is  no  place  within  a  sphere  of  a 
mile  in  diameter  where  the  candle  cannot  be  seen, 
that  is,  where  some  of  the  rays  from  the  small  flame 
will  not  be  found. 

J.  Why  do  you  limit  the  distance  to  half  a  mile  1 
T,  The  distance  of  course  will  be  greater  or  less 
according  to  the  size  of  the  candle  :  but  the  degree  of 
light,  like  heat,  diminishes  in  proportion  as  you  go 
farther  from  the  luminous  body. 

C.  Does  it  follow  the  same  law  as  gravity  ?* 
T.  It  does :  the  intensity  or  degree  of  light  de- 
creases as  the  square  of  the  distance  from  the  luminous 
body  increases. 

J,  Do  you  mean,  that  at  the  distance  of  two  yards 
from  a  candle  we  shall  have  four  times  less  light, 
than  we  should  have  if  we  were  only  one  yard 
from  it? 

T.  I  do  :  at  three  yards'  distance  nine  times  less 
light ;  and  at  four  yards'  distance  you  will  have  six- 
teen times  less  light  than  you  would  were  you  within 
a  yard  of  the  object. — I  have  one  more  thing  to  tell 
you  :  light  always  moves  in  straight  lines. 

/.  How  is  that  known  ? 

T,  Look  through  a  straight  tube  at  any  object,  and 
the  rays  of  light  will  flow  readily  from  it  to  the  eye ; 
but  let  the  tube  be  bent,  and  the  object  cannot  be  seen 
through  it,  which  proves  that  light  will  flow  only  in  a 
straight  line. 

This  is  plain  also  from  the  shadows  which  opaque 
bodies  cast ;  for  if  the  light  did  not  describe  straight 
lines,  there  would  be  no  shadow.    Hold  any  object 

*  See  Conver.  VH.  of  Mechanics. 


OF  RAYS  OF  LIGHT.  311 

"in  the  light  of  the  sun,  or  a  candle,  as  a  square  board 
or  book,  and  the  shadow  caused  by  it  proves  that 
light  moves  only  in  right  or  straight  lines;  for  the 
space  immediately  behind  the  object  is  in  shade. 


CONVERSATION  II. 

OF  RAYS  OF  LIGHT  OF  REFLECTION  AND  REFRACTION. 

C.  You  talked,  the  last  time  vi^e  met,  of  the  rays  of 
light  flowing  or  moving;  what  do  you  mean  by  a 
ray  of  light  ? 

T.  Light,  you  know,  is  supposed  to  be  made  up  of 
indefinitely  small  particles  ;  now  one  or  more  of  these 
I    particles,  in  motion  from  any  body,  is  called  a  ray  of 
j    light. — If  the  supposition  be  true,  that  light  does 
consist  of  particles  flowing  from  a  luminous  body,  as 
the  sun,  and  that  these  particles  are  about  eight 
minutes  in  coming  from  the  sun  to  us  ;  then,  if  the 
sun  were  blotted  from  the  heavens,  we  should  actually 
!    have  the  same  appearance  for  eight  minutes  after  the 
destruction  of  that  body  as  we  now  have. 

/.  I  do  not  understand  how  we  could  see  a  thing 
that  would  not  exist. 

r.  The  sun  is  perpetually  throwing  off  particles  of 
light,  which  travel  at  the  rate  of  twelve  millions  of 
j    miles  in  a  minute,  and  it  is  by  these  that  the  image  of 
I    the  body  is  impressed  on  our  eye.    The  sun  being 
i    blotted  from  the  firmament  would  not  affect  the 
'    course  of  the  particles  that  had  the  instant  before 
I    been  thrown  from  his  body ;  they  would  travel  on  as 
if  nothing  had  happened,  and  till  the  last  particles 
had  reached  the  eye  we  should  think  we  saw  the  sun 
I    as  much  as  we  do  now. 

C.  Do  we  not  actually  see  the  body  itself? 
i  T.  The  sense  of  sight  may,  perhaps,  not  be  un- 
aptly compared  to  that  of  smell  :  a  grain  of  musk  will 
I  throw  off  its  odoriferous  particles  all  round,  to  a  con- 
i  siderable  distance  ;  now  if  you  or  I  happen  to  be  near 
j    it,  the  particles  which  fall  upon  certain  nerves  in  the 


S42  OPTICS, 
nose,  will  excite  in  us  those  sensations  by  which  we 
say  we  have  the  smell  of  musk.  In  the  same  way 
particles  of  light  are  flowing  in  every  direction  from 
the  grain  of  musk,  some  of  which  fall  on  the  eye,  and 
these  excite  different  sensations,  from  which  we  say 
we  see  a  piece  of  musk. 

C.  But  the  musk  will  in  time  be  completely  dis- 
sipated, by  the  act  of  throwing  off  the  fine  particles  ; 
whereas  a  chair  or  a  table  may  throw  off  its  rays  so  as 
to  be  visible,  without  ever  diminishing  in  size. 

T.  True  :  because  whatever  is  distinguished  by  the 
sense  of  smell,  is  known  only  by  the  particles  of  the 
odoriferous  body  itself  flowing  from  it :  ^  whereas  a 
body  distinguished  by  the  sense  of  sight  is  known  by 
the  rays  of  light,  which  first  fall  on  the  body,  and  are 
then  reflected  from  it. 

/.  What  do  you  mean  by  being  reflected  ? 
T.  If  I  throw  this  marble  smartly  against  the 
wainscot,  will  it  remain  where  it  was  throwii  1 
J.  No:  it  will  rebound  or  come  back  again, 
r.  What  you  call  rebounding,  writers  on  optics 
denominate  reflection.    When  a  body  of  any  kind, 
whether  it  be  a  marble  with  which  you  play,  or  a 
particle  of  light,  strikes  against  a  surface,  and  is  sent 
back  again,  it  is  said  to  be  reflected.    If  you  shoot  a 
marble  straight  against  a  board,  or  other  obstacle,  it 
comes  back  in  the  same  line,  or  nearly  so  ;  but  sup- 
pose you  throw  it  sideways,  does  it  return  to  the 
hand]  .  , 

C.  Let  me  see  :  I  will  shoot  this  marble  against  the 
middle  of  one  side  of  the  room,  from  the  corner  of  the 
opposite  side. 

J.  You  see,  instead  of  coming  back  to  your  hand, 
it  goes  off  to  the  other  corner,  directly  opposite  to  the 
place  from  which  you  sent  it. 

r.  This  will  lead  us  to  the  explanation  of  one  of 
the  principal  definitions  in  optics,  viz.  that  the  angle  of 
reflection  is  always  equal  to  the  angle  of  incidence. 
You  know  what  an  angle  is  1* 

*  See  Convcr.  I.  of  ]\Ieclianics. 


INCIDENT  AND  REFLECTED  RAYS.  343 
C.  We  do :  but  not  what  an  angle  of  incidence  is. 
T.  I  said,  a  ray  of  light  was  a  particle  of  light  in 
motion:  now  there  are  incident  x^y  Sy^nA  reflected  x^ys. 
The  incident  rays  are  those  which  fall  on  the  sur- 
face ;  and  the  reflected  rays  are  those  which  are  sent  off 
from  it. 

C.  Does  the  marble  going  to  the  wamscot  represent 
the  incident  ray,  and  in  going  from  it  does  it  represent 
the  reflected  ray  ? 

r.  It  does  :  and  the  wainscot  may  be  called  the  re- 
flecting surface. 

J.  Then  what  are  the  angles  of  mcidence  and  re- 
flection ]  1 .  ,  1 

T.  Suppose  you  draw  the  Imes  on  which  the  niarble 
travelled,  both  to  the  wainscot,  and  from  it  again. 

C.  I  will  do  it  with  a  piece  of  chalk. 

T.  Now  draw  a  perpendicular*  from  the  point  where 
the  marble  struck  the  surface,  that  is,  where  your  two 
lines  meet. 

C.  I  see  there  are  two  angles,  and  they  seem  to  be 

^^^.^'We  cannot  expect  mathematical  precision  in 
such  trials  as  these  ;  but  if  the  experiment  were  accu- 
rately made,  the  two  angles  would  be  perfectly  equal : 
the  angle  contained  between  the  incident  ray,  and  the 
perpendicular,  is  called  the  angle  of  incidence,  and 
that  contained  between  the  perpendicular  and  reflected 
ray  is  called  the  angle  of  reflection. 

J.  Are  these  in  all  cases  equal,  shoot  the  marble  as 
you  will  1 

T.  They  are :  and  the  truth  holds  equally  with  rays 
of  light : — both  of  you  stand  in  front  of  the  looking- 
glass.  You  see  yourselves,  and  one  another  also  ;  for 
the  rays  of  light  flow  from  you  to  the  glass,  and  are 
reflected  back  again  in  the  same  lines.  Now  both  of 
you  stand  on  one  side  of  the  room.   ¥/hat  do  you  see? 

*  If  tlie  point  be  exactly  in  the  middle  of  one  side  of 
the  room,  a  perpendicular  is  readily  drawn  by  finding  the 
middle  of  the  opposite  side,  and  joining-  the  two  points. 


344  OFriCS. 

C.  Not  ourselves  j  but  the  furniture  on  the  opposite 
side. 

T,  The  reason  of  this  is,  that  the  rays  of  light  flowing 
from  you  to  the  glass  are  reflected  to  the  other  side  of 
the  room. 

C.  Then  if  I  go  to  that  part,  I  shall  see  the  rays  of 
light  flowing  from  my  brother : — and  I  do  see  him  in 
the  glass. 

J.  And  I  see  you. 

T.  Now  the  rays  of  light  flow  from  each  of  you  to 
the  glass,  and  are  reflected  to  one  another :  but  neither 
of  you  sees  himself. 

C.  No  :  I  will  move  in  front  of  the  glass,  now  I 
see  myself,  but  not  my  brother ;  and  I  think  I  under- 
stand the  subject  very  well. 

T.  Then  explain  it  to  me  by  a  figure,  which  you 
may  draw  on  the  slate. 

C.  Let  a  b  represent  the  looking- 
glass  :  if  I  stand  at  o,  the  rays  flow 
from  me  to  the  glass,  and  are  reflected 
back  in  the  same  line,  because  now 
there  is  no  angle  of  incidence,  and 
of  course  no  angle  of  reflection  ;  but        Fig.  1 . 
if  I  stand  at  .t,  then  the  rays  flow 
from  me  to  the  glass,  but  they  make  the  angle  .r  c  o, 
and  therefore  they  must  be  reflected  in  the  line 
c  y,  so  as  to  make  the  angle  y  c  o,  which  is  the  angle 
of  reflection,  equal  to  the  angle  x  c  o.    And  if  James 
stand  at  3/,  he  will  see  me  at  a-,  and  I  standing  at  x 
shall  see  him  at  y. 


CONVErxSATION  III. 

OF  THE   REFRACTION   OF  LIGHT. 

C.  If  glass  stop  the  rays  of  Hght,  and  reflect  them, 
why  cannot  I  see  myself  in  the  window  ? 

T.  It  is  the  silvering  on  the  glass  which  causes  the 
reflection,  otherwise  the  rays  would  pass  through  it 
without  being  stopped,  aijjl  jf  they  were  not  stopped 


REFRACTION  OF  LIGHT. 


343 


they  could  not  be  reflected.  No  glass  however  is  so 
transparent,  but  it  reflects  some  rays  :  put  your  hand 
to  within  three  or  four  inches  of  the  window,  and 
you  see  clearly  the  image  of  it. 

J.  So  I  do,  and  the  nearer  the  hand  is  to  the  glass 
the  more  evident  is  the  image  ;  but  it  is  formed  on  the 
other  side  of  the  glass,  and  beyond  it  too. 

T,  It  is;  this  happens  also  in  looking-glasses:  you 
do  not  see  yourself  on  the  surface,  but  apparently  as 
far  behind  the  glass,  as  you  stand  from  it  in  the  front. 

Whatever  suff'ers  the  rays  of  light  to  pass  through 
it  is  called  a  medium.  Glass,  which  is  transparent,  is 
a  medium;  so  also  is  air;  water,  and  indeed  all  fluids 
that  are  transparent  are  called  media, ^  and  the  more 
transparent  the  body,  the  more  perfect  is  the  medium, 

C.  Do  the  rays  of  light  pass  through  these  in  a 
straight  line  1 

I  T.  They  do :  but  not  in  precisely  the  same  direc- 
tlon  in  which  they  were  moving  before  they  entered  it. 

:  They  are  be7it  out  of  their  former  course,  and  this  is 

i   called  refraction, 

J.  Can  you  explain  this  term  more  clearly  1 

J      T.  Suppose  d  /  to  be  a 
piece  of  glass  two  or  three 

!  inches  thick;  and  a  ray  of 
light  c  a  to  fall  upon  it  at  a ; 

j  it  will  not  pass  through  it  in 

j  the  direction  c  s,  but  when  it 

i  comes  to  a  it  will  be  bent  to- 
wards the  perpendicular  a  b,  Fig.  2. 
and  go  through  the  glass  in 

the  course  a  x,  and  when  it  comes  into  the  air  it  will 
pass  on  in  the  direction  x  z,  which  is  parallel  to  c  s. 
C.  Does  this  happen  if  the  ray  fall  perpendicularly 
!    on  the  glass,  as  p  a  f 

I  T.  In  that  case  there  is  no  refraction,  but  the  ray 
proceeds  in  its  passage  through  the  glass,  precisely  in 
the  same  direction  as  it  did  before  it  entered  it,  namely, 
in  the  direction  p  b. 

Q  2 


346  OPTICS. 

J.  Refraction  then  takes  place  only  when  the  rays 
fall  obliquely  or  slantwise  on  the  medium  ? 

T.  Just  so :  rays  of  light  may  pass  out  of  a  rarer 
into  a  denser  medium,  as  from  air  into  water  or  glass: 
or  they  may  pass  from  a  denser  medium  into  a  rarer,  as 
from  water  into  air. 

C.  Are  the  effects  the  same  in  both  cases  ? 

r.  They  are  not ;  and  I  wish  you  to  remember  the 
difference.  When  light  passes  out  of  a  rarer  into  a 
denser  medium,  it  is  drawn  to  the  perpendicular  ;  thus 
if  c  a  pass  from  air  into  glass,  it  moves,  in  its  passage 
through  it,  in  the  line  a  x,  which  is  nearer  to  the  per- 
pendicular a  b  than  the  line  a  s  which  was  its  first 
direction. 

But  when  a  ray  passes  from  a  denser  medium 
into  a  rarer,  it  moves  in  a  direction  farther  from  the 
perpendicular ;  thus  if  the  ray  x  a  pass  through  glass 
or  water  into  air,  it  will  not  when  it  comes  to  a  move 
in  the  direction  a  m,  but  in  the  line  a  c,  which  is  farther 
than  a  m  from  the  perpendicular  a  p, 

J.  Can  you  shew  us  any  experiment  in  proof  of  this? 

T.  Yes,  I  can:  here  is  a  common  earthen  pan,  on 
the  bottom  of  which  I  will  lay  a  shilUng,  and  will 
fasten  it  with  a  piece  of  soft  wax,  so  that  it  shall  not 
move  from  its  place,  while  I  pour  in  some  water.  Stand 
back  till  you  j  ust  lose  sight  of  the  shilling. 

J.  The  side  of  the  pan  now  completely  hides  the 
sight  of  the  money  from  me. 

T.  I  will  pour  in  a  pitcher  of  clear  water. 

J.  I  now  see  the  shilling :  how  is  this  to  be  ex- 
plained 1 

T.  Look  to  the  last  figure,  and  conceive  your  eye 
to  be  at  r,  a  b  the  side  of  the  pan,  and  the  piece  of 
money  to  be  at  x :  now  when  the  pan  is  empty,  the  rays 
of  light  flow  from  x  in  the  direction  .t  a  m,  but  your 
eye  is  at  c,  of  course  you  cannot  see  any  thing  by  the 
ray  proceeding  along  x  am.  As  soon  as  I  put  the 
water  into  the  vessel,  the  rays  of  light  proceed  from 
X  to  tJ,  but  there  they  enter  from  a  denser  to  a  rarer 


REFRACTION  OF  LIGHT.  347 
medium;  and,  therefore,  instead  of  moving  in  a  m,  as 
they  did  when  there  was  no  water,  they  will  be  bent 
from  the  perpendicular,  and  will  come  to  your  eye  at 
c,  as  if  the  shilling  were  situate  at  n, 

J,  And  it  does  appear  to  me  to  be  at 

T,  Remember  what  I  am  going  to  tell  you,  for  it 
is  a  sort  of  axiom  in  optics  :  "We  see  every  thing 
in  the  direction  of  that  line  in  which  the  rays  approach 
us  last."  Which  may  be  thus  illustrated  :  I  place  a 
candle  before  the  looking-glass,  and  if  you  stand  also 
before  the  glass  the  image  of  the  candle  appears  behind 
it ;  but  if  another  looking-glass  be  so  placed  as  to  re- 
ceive the  reflected  rays  of  the  candle,  and  you  stand 
before  this  second  glass,  the  candle  will  appear  behind 
that;  because  the  mind  transfers  every  object  seen 
along  the  line  in  which  the  rays  came  to  the  eye  last. 

C.  If  the  shilling  were  not  moved  by  the  pouring 
in  of  the  water,  I  do  not  understand  how  we  could  see 
it  afterwards. 

T.  But  you  do  see  it  now  at  the  point  r?,  or  rather  at 
the  little  dot  just  above  it,  which  is  an  inch  or  two  from 
the  place  where  it  was  fastened  to  the  bottom,  and 
from  which  you  may  convince  yourself  it  has  not  moved. 

/.  1  should  like  to  be  convinced  of  this  :  will  you 
make  the  experiment  again  that  I  may  be  satisfied  of  it? 

T.  You  may  make  it  as  often  as  you  please,  and 
the  effect  will  always  be  the  satne  :  but  you  must  not 
imagine  that  the  shilling  only  will  appear  to  move,  the 
bottom  of  the  vessel  seems  also  to  change  its  place. 

J.  It  appears  to  me  to  be  raised  higher  as  the  water 
is  poured  in. 

T,  I  trust  you  are  satisfied  by  this  experiment :  but 
I  can  shew  you  another  equally  convincing ;  but  in 
this  we  stand  in  need  of  the  sun.^ 

Take  an  empty  bason  or  pan  a  into  a  dark 
room,  having  only  a  very  small  hole  in  the  a^/^ 
window  shutter :  so  place  the  bason  that  a  p/j 
ray  of  light  s  s  shall  fall  upon  the  bottom  of  |/j 
it  at  a ;  here  I  make  a  small  mark,  and  then  ^ 
fill  the  bason  with  water.    Now  where  does    y^^.  ^ 
the  ray  fain 


348  OPTICS. 

J,  IVIuch  nearer  to  the  side  at  h. 

T.  I  did  not  move  ihe  bason,  and  therefore  could 
Iiave  no  power  in  altering  the  course  of  the  light. 

C.  It  is  very  clear  that  the  ray  was  refracted  by  the 
water  at  s :  and  I  see  that  the  effect  of  refraction  in 
this  instance  has  been  to  draw  the  ray  nearer  to  a  per- 
pendicular, which  may  be  conceived  to  be  the  side  of 
the  vessel. 

T.  The  same  thing  may  be  shewn  wjth  a  candle  in 
a  room  otherwise  dark ;  let  it  stand  in  such  manner  as 
that  the  shadow  of  the  side  of  a  pan  or  box  may  fall 
somewhere  at  the  bottom  of  it;  mark  the  place,  and 
pour  in  water,  and  the  shadow  will  not  then  fall  so  far 
from  the  side. 


CONVERSATION  IV. 

OF  THE   REFLECTION  AND   REFRACTION  OF  LIGHT. 

T.  We  will  proceed  to  some  farther  illustrations  of 
the  laws  of  reflection  and  refraction.  We  shut  out  all 
the  light  except  the  ray  that  comes  in  at  the  small  hole 
in  the  shutter :  at  the  bottom  of  this  bason,  where  the 
ray  of  light  falls,  I  lay  this  piece  of  looking-glass;  and 
if  the  water  be  rendered  in  a  small  degree  opaque  by 
mixing  with  it  a  few  drops  of  milk,  and  the  room  be 
filled  with  dust  by  sweeping  a  carpet,  or  any  other 
means,  then  you  will  see  the  refraction  which  the  ray 
from  the  shutter  undergoes  in  passing  into  the  water, 
the  reflection  of  it  at  the  surface  of  the  looking-glass, 
and  the  refraction  which  takes  place  when  the  ray 
leaves  the  water,  and  passes  again  into  the  air.  _ 

J.  Does  this  refraction  take  place  in  all  kinds  of 
glass  ? 

T.  It  does  ;  but  where  the  glass  is  very  thin,  as  in 
window  glass,  the  deviation  is  so  small  as  to  be  gene- 
rally overlooked.  You  may  now  understand  why  the 
oar  in  the  vvalei  appears  bent,  though  it  be  really 


OPTICAL  DECEPTIONS.  349 

straight;  for  suppose  a  b  be 
water,  and  max  the  oar, 
the  image  of  the  part  a  co 
in  the  water  will  lie  above 
the  object,  so  that  the  oar 
will  appear  in  the  shape  ^ 
«i « /Zjinstead  of  m  a  a:.  On 

this  account  also  a  fish  in  the  water  appears  nearer  the 
surface  than  it  actually  is,  and  a  marksman  shooting 
at  it  must  aim  below  the  place  which  it  seems  to  occupy. 

C,  Does  the  image  of  the  object  seen  in  the  water 
always  appear  higher  than  the  object  really  is  1 

T.  It  appears  one  fourth  nearer  the  surface  than  the 
object  is.  Hence  a  pond  or  river  is  a  third  part  deeper 
than  it  appears  to  be,  which  is  of  importance  to  re- 
member, for  many  a  school-boy  has  lost  his  life  by 
imagining  the  water  into  which  he  plunged  was  within 
his  depth. 

J.  You  say  the  bottom  appears  one  fourth  nearer 
the  surface  than  it  is ;  and  then  that  the  water  is  a 
third  deeper  than  it  seems  to  be :  I  do  not  understand 
this. 

T.  Suppose  the  river  to  be  six  feet  deep,  which  is 
sufficient  to  drown  you  or  me,  if  we  cannot  swim :  I 
say  the  bottom  will  appear  to  be  only  four  feet  and  a 
half  from  the  surface,  in  which  case  you  could  stand 
and  have  the  greater  part  of  your  head  above  it :  of 
course  it  appears  to  be  a  foot  and  a  half  shallower  than 
it  is,  but  a  foot  and  a  half  is  just  the  third  part  of  four 
feet  and  a  half. 

C.  Can  this  be  shewn  by  experiment  1 

T,  It  may  :  I  take  this  large  empty  pan,  and  with 
a  piece  of  soft  wax  stick  a  piece  of  money  at  the  bot- 
tom, but  so  that  you  can  just  see  it  as  you  stand  ; 
keep  your  position,  and  I  will  pour  in  a  quantity  of 
water  gradually,  and  tell  me  the  appearance. 

C.  The  shilling  rises  exactly  in  the  same  proportion 
as  you  pour  in  the  water. 

T.  Recollect,  then,  in  future,  that  we  cannot  judge 
of  distances  so  well  in  water  as  in  air. 


350 


OPTICS. 


J.  And  I  am  sure  we  cannot  of  magnitudes  :  for 
in  looking  through  the  sides  of  a  globular  glass  at  some 
gold  and  silver  fish,  I  thought  them  very  large,  but  if 
1  looked  down  upon  them  from  the  top  they  appeared 
much  smaller  indeed. 

r.  Here  the  convex  or  round  shape  of  the  glass 
becomes  a  magnifier,  the  reason  of  vi^hich  will  be  ex- 
plained hereafter.  A  fish  will,  however,  look  larger 
in  water  than  it  really  is. — I  will  shew  you  another  ex- 
periment which  depends  on  refraction  :  here  is  a  glass 
goblet  two-thirds  full  of  water  ;  I  throw  into  it  a  shil- 
ling, and  place  a  plate  on  the  top  of  it,  and  turn  it 
quickly  over,  that  the  water  may  not  escape.  What 
do  you  see  ? 

C.  There  is  certainly  a  half-crown  lying  on  the 
plate,  and  a  shilling  seems  swimming  above  it  in  the 
water. 

r.  So  it  appears  indeed ;  but  it  is  a  deception,  which 
arises  from  your  seeing  the  piece  of  money  in  two 
directions  at  once,  viz.  through  the  conical  surface  of 
the  water  at  the  side  of  the  glass,  and  through  the  flat 
surface  at  the  top  of  the  water.  The  conical  surface, 
as  was  the  case  with  the  globular  one  in  which  the 
fish  were  swimming, magnifies  the  money;  but  by  the 
flat  surface  the  rays  are  only  refracted,  on  which  ac- 
count the  money  is  seen  higher  up  in  the  glass,  and 
of  its  natural  size,  or  nearly  so. 

T.  If  1  look  side-ways  at  thje  money  I  only  see  the 
large  piece ;  and  if  only  at  top,  I  see  it  in  its  natural 
size  and  state. 

C.  Look  again  at  the  fish  in  the  glass,  and  you  will 
see  through  the  round  part  two  very  large  fish,  and 
seeing  them  from  the  upper  part  they  appear  of  their 
natural  size;  the  deception  is  the  same  as  with  the 
shilling  in  the  goblet. 

I'.  The  principle  of  refraction  is  productive  of  some 
very  important  effects.  By  this  the  sun  every  clear 
morning  is  seen  several  minutes  before  he  comes  to 
the  horizon,  and  as  long  after  he  sinks  beneath  it  in  the 
evening. 


REFRACTION  OF  THE  ATMOSPHERE.  351 

C.  Then  the  days  are  longer  than  they  would  be  if 
there  was  no  such  thing  as  refraction.  Will  you  ex- 
plain how  this  happens? 

T.  I  will :  you  know  we  are  surrounded  with  an  at- 
mosphere which  extends  all  round  the  earth,  and  above 
it  to  about  the  height  of  forty-five  miles ;  now  the  dot- 
ted part  of  this  figure  represents  that 
atmosphere  :  suppose  a  spectator  no  <> « 

stand  at  s,  and  the  sun  to  be  at  a  3  if  \  i 

there  were  no  refraction  the  person     /  f 

at  s  would  not  see  the  rays  from  the  ^""^,,53^^ 

sun  till  he  were  situate  with  regard 

to  the  sun  in  a  line  s  x  a,  because  Fio",  5. 

when  it  was  below  the  horizon  at  6, 

the  rays  would  pass  by  the  earth  in  the  direction  bxz; 

but  owing  to  the  atmosphere,  and  its  refracting  power, 

when  the  rays  from  b  reach  x,  they  are  bent  towards 

the  perpendicular,  and  carried  to  the  spectator  at  s. 

J.  Will  he  really  see  the  image  of  the  sun  while  it 
is  below  the  horizon? 

T,  He  will ;  for  it  is  easy  to  calculate  the  moment 
when  the  sun  should  rise  and  set,  and  if  that  be  com- 
pared with  exact  observation,  it  will  be  found  that  the 
image  of  the  sun  is  seen  sooner  and  later  than  this  by 
several  minutes  every  clear  day, 

C.  Are  we  subject  to  the  same  kind  of  deception 
when  the  sun  is  actually  above  the  horizon  ? 

T,  We  are  always  subject  to  it  in  these  latitudes, 
and  the  sun  is  never  in  that  place  in  the  heavens  where 
he  appears  to  be. 

Why  in  these  latitudes  particularly? 

T.  Because  with  us  the  sun  is  never  in  the  zenith, 
or  directly  over  our  heads;  and  in  that  situation  alone 
his  true  place  in  the  heavens  is  the  same  as  his  appa- 
rent place. 

C.  Is  that  because  there  is  no  refraction  when  the 
rays  fall  perpendicularly  on  the  atmosphere  ? 

T.  It  is :  but  when  the  sun  is  at  m,  his  rays  will  not 
proceed  in  a  direct  line  m  0  r,  but  will  be  bent  out  of 
their  course  at  0,  and  go  in  the  direction  o  s,  and  the 


352 


OPTICS. 


spectator  will  imagine  he  sees  the  sun  in  the  line 
s  0  n. 

C.  What  makes  the  moon  look  so  much  larger  when 
it  is  just  above  the  horizon,  than  when  it  is  higher  up? 

r.  The  thickness  of  the  atmosphere  when  the  moon 
is  near  the  horizon,  renders  it  less  bright  than  when  it 
is  higher  up,  which  leads  us  to  suppose  it  is  farther  off 
in  the  former  case  than  in  the  latter ;  and,  because  we 
imagine  it  to  be  farther  from  us,  we  take  it  to  be  a 
larger  object  than  when  it  is  higher  up. 

It  is  owing  to  the  atmosphere  that  the  heavens  ap- 
pear bright  in  the  day-time.  Without  any  atmosphere 
only  that  part  of  the  heavens  would  appear  luminous 
in  which  the  sun  is  placed  ;  in  that  case,  if  we  could 
live  without  air,  and  should  stand  with  our  backs  to 
the  sun,  the  whole  heavens  would  appear  as  dark  as 
night. 

CONVERSATION  V. 

DEFINITIONS  OF  THE  DIFFERENT  KINDS  OF  LENSFS  

OF  MR.  Parker's  burning  lens,  and  the  effects 

PRODUCED  EY  IT. 

T.  I  must  claim  your  attention  to  a  few  other  defi- 
nitions ;  the  knowledge  of  which  will  be  wanted  as  we 
proceed, 

A  pencil  of  rays  is  any  number  that  proceed  from  a 
point. 

Parallel  rays  are  such  as  move  always  at  the  same 
distance  from  each  other. 

C.  That  is  something  like  the  definition  of  paralUd 
lines.*  But,  when  you  admitted  the  rays  of  light 
through  the  small  hole  in  the  shutter,  they  did  not 
seem  to  flow  from  that  point  in  parallel  lines,  but  to 
recede  from  each  other  in  proportion  to  their  distance 
from  that  point. 

T,  They  did ;  and  when  they  do  thus  recede  from 

*  Parallel  lines  are  tliosc  which  being'  infinitely  ex- 
tended never  meet. 


OF  THE  DIFFERENT  LENSES. 


353 


FiP  .  6. 


each  other,  as  in  this  figure  from 
c  to  cd,  then  they  are  said  to 
diverge.  But  if  they  continu- 
ally approach  towards  each  other 
as  in  moving  from  cdto  c,  they 
are  said  to  converge. 

J.  What  does  the  dark  part 
of  this  figure  represent  ? 

T.  It  represents  a  glass  lens,  of  which  there  are 
several  kmds. 

C.  How  do  you  describe  a  lens  ? 

T.  A  le7is  is  a  glass  ground  into  such  a  form  as  to 
collect  or  disperse  the  rays  of  light  which  pass  through 
It.  Lenses  are  of  different  shapes  from  which  they 
take  their  names.    They  are  represented  here  in  one 


Fig.  7. 

view,  a  is  such  a  one  as  that  in  the  last  figure,  and 
It  is  called  a  plano-convex,  because  one  side  is  flat, 
and  the  other  convex  ;  6  is  a  plano-concave,  one  side 
being  flat,  and  the  other  is  coyicave ;  c  is  a  double 
convex-lens,  because  both  sides  are  convex;  d  is  a 
double  concave,  because  both  sides  are  concave ;  and 
e  IS  called  a  meniscus,  being  convex  on  one  side',  and 
concave  on  the  other;  of  this  kind  are  all  watch 
glasses. 

/.  I  can  easily  conceive  of  diverging  rays,  or  rays 
proceeding  from  a  point ;  but  what  is  to  make  them 
converge,  or  come  to  a  point  1 

T.  Look  again  to  Fig.  6 ;  now  a,  b,  m,  &c.  re- 
present parallel  rays,  falling  upon  a  convex  surface, 
of  glass  for  instance,  all  of  which,  except  the  middle 
one,  fall  upon  it  obliquely,  and,  according  to  what 
we  saw  yesterday,  will  be  refracted  towards  the  per- 
pendicular. 


354  OPTluS. 

C.  And  I  suppose  they  will  all  meet  in  a  ccitain 
point  in  that  middle  line. 

T,  That  point  c  is  called  the/ocits  :  the  dark  part 
of  this  figure  only  represents  the  glass,  at  cdn. 

C.  Have  you  drawn  the  circle  to  shew  the  exact 
curve  of  the  different  lenses  1 

2\  Yes  :  and  you  see  that  parallel  rays  falling 
upon  a  plano-convex  lens  meet  at  a  point  behind  it, 
the  distance  of  which  from  the  middle  of  the  glass 
is  exactly  equal  to  the  diameter  of  the  sphere  of  which 
the  lens  is  a  portion. 

J.  And  in  the  case  of  a  double 
■  convex,  is  the  distance  of  the 
focus  of  parallel  rays,  equal 
only  to  the  radius  of  the  sphere? 

T.  It  is :  and  you  see  the 
reason  of  it  immediately,  for 
two  concave  surfaces  have  dou-  Fig.  8, 

ble  the  effect  in  refracting  rays 
to  what  a  single  one  has  :  the  latter  bringing  them 
to  a  focus  at  the  distance  of  the  diameter,  the  former 
at  half  that  distance,  or  of  the  radius. 

C.  Sometimes,  perhaps,  the  two  sides  of  the  same 
lens  may  have  different  curves  :  what  is  to  be  done 
then? 

T.  If  you  know  the  radius  of  both  the  curves,  the 
following  rule  will  give  you  the  answer  : — 

As  the  sum  of  the  radii  of  both  curves  or  con- 
vexities is  to  the  radius  of  either,  so  is  double  the 
radius  of  the  other  to  the  distance  of  the  focus  from 
the  middle  point." 

J,  Then  if  one  radius  be  four  inches,  and  the  other 

24  3 

three  inches,  I  say,  as  4  x  3:4::6:—  =  3-^ » 

or  to  nearly  three  inches  and  a  half. — I  saw  a  gentle- 
man lighting  his  pipe  yesterday  by  means  of  the  sun's 
rays  and  a  glass ;  was  that  a  double  convex  lens  1 

T.  I  dare  say  it  was  :  and  you  now  sec  the  reason 
of  what  then  you  could  not  comprehend  :  all  the 
rays  of  the  sun  that  fall  on  the  surface  of  the 


OF  THE  FOCAL  DISTANCE.  355 

glass  (see  Fig.  8.)  are  collected  in  the  point/,  which, 
in  this  case,  may  represent  the  tobacco  in  the  pipe. 

C,  How  do  j^ou  calculate  the  heat  which  is  col- 
lected in  the  focus  1 

T.  The  force  of  the  heat  collected  in  the  focus  is 
in  proportion  to  the  common  heat  of  the  sun,  as  the 
area  of  the  glass  is  to  the  area  of  the  focus  :  of  course 
it  may  be  a  hundred  or  even  a  thousand  times  greater 
in  the  one  case  than  in  the  other. 

J,  Have  I  not  heard  you  say  that  Mr.  Parker,  of 
Fleet  Street,  made  once  a  very  large  lens,  which  he 
used  as  a  burning-glass  ? 

T.  He  formed  one  three  feet  in  diameter,  and 
when  fixed  in  its  frame  it  exposed  a  clear  surface  of 
more  than  two  feet  eight  inches  in  diameter,  and  its 
focus,  by  means  of  another  lens,  was  reduced  to  a 
diameter  of  half  an  inch.  The  heat  produced  by 
this  was  so  great  that  iron  plates  were  melted  in  a 
few  seconds  : — tiles  and  slates  became  red  hot  in  a 
moment,  and  were  vitrified,  or  changed  into  glass  : — 
sulphur,  pitch,  and  other  resinous  bodies  were  melted 
under  water  : — wood-ashes,  and  those  of  other  vege- 
table substances,  were  turned  in  a  moment  into  tran- 
sparent glass. 

C.  Would  the  heat  produced  by  it  melt  all  the 
metals  1 

T,  It  would :  even  gold  was  rendered  fluid  in  a 
few  seconds  ;  notwithstanding,  however,  this  intense 
heat  at  the  focus,  the  finger  might  without  the  small- 
est injury  be  placed  in  the  cone  of  rays  within  an 
inch  of  the  focus. 

J.  There  was,  however,  I  should  suppose,  some 
risk  in  this  experiment,  for  fear  of  bringing  the  finger 
too  near  the  focus. 

T,  Mr.  Parker's  curiosity  led  him  to  try  what  the 
sensation  would  be  at  the  focus,  and  he  describes  it 
like  that  produced  by  a  sharp  lancet,  and  not  at  all 
similar  to  the  pain  produced  by  the  heat  of  fire  or  a 
candle.  Substances  of  a  white  colour  were  difficult 
to  be  acted  upon. 


350  OPTICS. 

C.  I  suppose  he  could  make  water  boil  in  a  very 
short  time  with  the  lens. 

r.  If  the  water  be  very  clear,  and  contained  in  a 
clear  glass  decanter,  it  will  not  be  warmed  by  the 
most  powerful  lens.  But  a  piece  of  wood  may  be 
burned  to  a  coal,  when  it  is  contained  in  a  decanter 
of  water. 

J.  Will  not  the  heat  break  the  glass  ? 

r.  It  will  scarcely  warm  it :  iff  however,  a  piece 
of  metal  be  put  in  the  water,  and  the  point  of  rays  be 
thrown  on  that,  it  will  communicate  heat  to  the 
water,  and  sometimes  make  it  boil.  The  same  effect 
will  be  produced  if  there  be  some  ink  thrown  into  the 
water. 

If  a  cavity  be  made  in  a  piece  of  charcoal,  and  the 
substance  to  be  acted  on  be  put  in  it,  the  effect  pro- 
duced by  the  lens  will  be  much  increased.  Any  metal 
thus  enclosed  melts  in  a  moment,  the  fire  sparkling 
like  that  of  a  forge  to  which  the  blast  of  a  bellows  is 
applied. 


CONA^EKSATION  VI. 

OF  PARALLEL  RAYS  OF  DIVERGING  AND  CON- 
VERGING RAYS  or  THE  EOCUS  AND  EOCAL  DIS- 
TANCES. 

C.  I  have  been  looking  at  the  figures  6  and  8,  and 
see  that  the  rays  falling  upon  the  lenses  are  parallel 
to  one  another  :  are  the  sun's  rays  parallel  1 

T,  They  are  considered  so  :  but  you  must  not 
suppose  that  all  the  rays  wliich  come  from  the  surface 
of  an  object,  as  the  sun,  or  any  other  body,  to  the  eye, 
are  parallel  to  each  other,  but  it  must  be  understood 
of  those  rays  only  which  proceed  from  a  single  pomt. 
Suppose  s  to  be  the  sun,  the 

rays  which  proceed  from  a  ^^y^^^^^^-f^tr;:^:^^^^^ 
single  point  a,  do  in  reality     '"c         '  Ji 
form  a  cone,    the   base  of  Fig.  9. 


OF  PARALLEL  RAYS.  31)7 
which  is  the  pupil  of  the  eye,  and  its  height  is  the 
distance  from  us  to  the  sun. 

/ .  B\it  the  breadth  of  the  eye  is  nothing  when  com- 
pared to  a  line  ninety-five  millions  of  miles  long. 

T.  And  for  that  reason  the  various  rays  that  pro- 
ceed from  a  single  point  in  the  sun  are  considered  as 
parallel,  because  their  inclination  to  each  other  is  in- 
sensible. The  same  may  be  said  of  any  other  point, 
as  c.  Now  all  the  rays  that  we  can  admit  by  means 
of  a  small  aperture  or  hole,  must  proceed  from  an 
indefinitely  small  point  of  the  sun,  and  therefore  they 
are  justly  considered  as  parallel. 

If  now  we  take  a  ray  from  the  point  a,  and  another 
from  c,  on  opposite  points  of  the  sun's  disk,  they  will 
form  a  sensible  angle  at  the  eye  ;  and  it  is  from  this 
angle  a  eg  that  we  judge  of  the  apparent  size  of  the 
sun,  which  is  about  half  a  degree  in  diameter. 

C.  Will  the  size  of  the  pupil  of  the  eye  make  any 
difference  with  regard  to  the  appearance  of  the  ob- 
ject? 

T.  The  larger  the  pupil  the  brighter  will  the  object 
appear,  because  the  larger  the  pupil  is  the  greater  num- 
ber of  rays  it  will  receive  from  any  single  point  of  the 
object. — And  I  wish  you  to  remember  what  I  have 
told  you  before,  that  whenever  the  appearance  of  a 
given  object  is  rendered  larger  and  brighter,  we  always 
imagine  that  the  object  is  nearer  to  us  than  it  really 
is,  or  than  it  appears  at  other  times. 

J.  If  there  be  nothing  to  receive  the  rays  (Fig.  8.) 
at  fy  would  they  cross  one  another  and  diverge  ? 

T.  Certainly,  in  the  same  manner  as  they  con- 
verged in  coming  to  it ;  and  if  another  glass,  fg,  of 
the  same  convexity  as  d  e,  be  placed  in  the  rays  at  the 
same  distance  from  the  focus,  it  will  so  refract  them, 
that,  after  going  out  of  it,  they  will  be  parallel,  and 
so  proceed  on  in  the  same  manner  as  they  came  to 
the  first  glass. 

C.  There  is,  however,  this  difference — all  the  rays 
except  the  middle  one  have  changed  sides. 

T.  You  are  right  j  the  ray  b,  which  entered  at 


358 


OPTICS. 


bottom,  goes  out  at  the  top  h;  and  a,  which  entered 
at  the  top,  goes  out  at  the  bottom  c,  and  so  of  the 
rest. 

If  a  candle  be  placed  at/,  the  focus  of  the  convex 
glass,  the  divergmg  rays  in  the  space  f/ g  will  be  so 
refracted  by  the  glass,  that  after  going  out  of  it,  they 
will  become  parallel  again. 

J.  What  will  be  the  effect  if  the  candle  be  nearer 
to  the  glass  than  the  point/? 

r.  In  that  case,  as  if  the  candle 
be  at  g,  the  rays  will  diverge  after 
they  have  passed  through  the 
glass,  and  the  divergency  will  be 
greater  or  less  in  proportion  as  the 
candle  is  more  or^less  distant  from 
the  focus. 

C.  If  the  candle^be  placed  far- 
ther from  the  lens  tten  the  focus/,  will  the  rays  meet 
in  a  point  after  they  have  passed,  through  iti 

T.  They  will :  thus,  if  the 
candle  be  placed  at  g,  the  rays, , 
after  passing  the  lens,  will  meet 
in  X ;  and  this  point  x  will  be  moj^e 
or  less  distant  from  the  glass,  as 
the  candle  is  nearer  to,  or  far- 
ther from,  its  focus. — Where 
the  rays  meet  they  form  an 
inverted  image  of  the  flame  of  the  candle. 
/.  Why  so? 

T.  Because  that  is  the  point  where  the  rays,  if 
they  are  not  stopped,  cross  each  other  :  to  satisfy  you 
on  this  head  I  will  hold  in  that  point  a  sheet  of  paper, 
and  you  now  see  that  the  flame  of  the  candle  is  in- 
verted. 

This  may  be  explained  in  the  following  manner  : 
Let  a  be  represent  an  arrow  placed  beyond  the  focus 
g  of  a  double  convex  lens,  def;  some  rays  will  flow 
from  every  part  of  the  arrow,  and  fall  on  the  lens  ; 
but  we  shall  consider  only  those  which  flow  from  the 
points  a,  h,  and  c.    The  rays  which  come  from  a,  as 


Fi.cr.  11. 


OF  THE  INVERTED  IMAGE. 


359 


Fig.  12. 


ad,  ae,  and  a/,  will  be  refracted  by  the  lens,  and 
meet  in  a.  Those  which  come  from  h,  as  hd^  be,  and 
h f,  will  unite  in  b,  and  those  which  come  from  c  will 
unite  m  c.  ' 

C.  I  see  clearly  how  the  rays  from  h  are  refracted, 
and  unite  in  b  ;  but  it  is  not  so  evident  with  regard  to 
those  from  the  extremities  a  and  c. 

T.  I  admit  it,  but  yau  must  remember  the  difficulty 
consists  in  this,  the  rays  fall  mote;  obliquely  on  the 
glass  from  those  points  than  from  the  middle,  and 
therefore  the  refraction  is' very  different.  The  ray 
i-n  the  centre  suffers  no  refraction,  hdis  refracted  into 
B :  and  if  another  ray  went  from  i  as  id,  it  would  be 
refracted  to  i  somewhere  between  b  and  a,  and  the 
rays  from  a  must  for  the  same  reason  be  refracted 
to  A. 

J.  If  the  object  a 6c  is  brought  nearer  to  the  glass, 
will  the  picture  be  removed  to  a  greater  distance  1 

r.  It  will :  for  then  the  rays  will  fall  more  diver- 
ging upon  the  glass,  and  cannot  be  so  soon  collected 
into  the  corresponding  points  behind  it. 

C  From  what  you  have  said,  I  see  that  if  the  ob- 
ject a  ft  c  be  placed  in  g,  the  rays,  after  refraction,  will 
go  out  parallel  to  one  another  ;  and  if  brought  nearer 
to  the  glass  than  g,  then  they  will  diverge  from  one 
another,  so  that  in  neither  case  an  image  will  be 
formed  behind  the  lens. 

J.  To  get  an  image,  must  the  object  be  beyond  the 
focus  g  ? 

T,  It  must :  and  the  picture  will  be  bigger  or  less 
than  the  object,  as  its  distance  from  the  glass  is 


360 


OPTICS 


greater  or  less  than  the  distance  of  the  object ;  if 
abc  (Fig.  12.)  be  the  object,  c  b  a  will  be  the  picture  ; 
and  if  c  B  A  be  the  object,  abc  will  be  the  picture. 

C.  Is  there  any  rule  to  find  the  distance  of  the 
picture  from  the  glass  1 

T,  If  you  know  the  focal  distance  of  the  glass, 
and  the  distance  of  the  object  from  the  glass,  the 
rule  is  this : 

"  Multiply  the  distance  of  the  focus  by  the  distance 
of  the  object,  and  divide  the  product  by  their  differ- 
ence, the  quotient  is  the  distance  of  the  picture." 

J.  If  the  focal  distance  of  the  glass  be  seven 
inches,  and  the  object  be  nine  inches  from  the  lens, 

I  say,  Z-^  =  ^=:31|  inches;  of  course  the  pic- 

ture  will  be  very  much  larger  than  the  object.  For, 
as  you  have  said,  the  picture  is  as  much  bigger  or  less 
than  the  object,  as  its  distance  from  the  glass  is  greater 
or  less  than  the  distance  of  the  object, 

r.  If  the  focus  be  seven  inches,  and  the  object  at 
the  distance  of  seventeen  inches,  then  the  distance 

of  the  picture  will  be  found  thus  ^        =  =12 

inches  nearly. 


CONVERSATION  VII. 

IMAGES  OF  OBJECTS  INVERTED  OF  THE  SCIOPTRIC 

BALL  OF  LENSES  AND  THEIR  FOCI. 

J.  Will  the  image  of  a  candle  when  received 
through  a  convex  lens  be  inverted  1 

T.  It  will,  as  you  shall  see  :  here  is  no  light  in  this 
room  but  from  the  candle,  the  rays  of  which  pass 
through  a  convex  lens,  and  by  holding  a  sheet  of 
paper  in  a  proper  place,  you  will  see  a  complete  in- 
verted image  of  the  candle  on  it. 

An  object  seen  through  a  very  small  aperture 
appears  also  inverted,  but  it  is  very  imperfect  com- 


OF  INVERTED  IMAGES.  3g| 
pared  to  an  image  formed  with  the  lens  ;  it  is  faint  for 
want  of  light,  and  it  is  confused  because  the  rays  in- 
terfere with  one  another.  ^ 
C.  What  is  the  reason  of  its  being  inverted 
J .  Because  the  rays  from  the  extreme  parts  of  the 
object  must  cross  at  the  hole.  If  you  look  through  a 
very  sjiall  hole  at  any  object,  the  object  appears 
magnified  Make  a  pm-hole  in  a  she^t  of  brown 
paper,  and  look  through  it  at  the  small  print  of  this 

J.  It  is,  indeed,  very  much  magnified. 
imLA^        f^^""^-  ^PP^^aches  a  convex  lens,  its 
mage  departs  from  it ;  and  as  the  object  recedes,  its 
image  advances.    Make  the  experimtnt  with  a  can 
aie  and  a   ens,  properly  mounted,  in  a  long  room: 

theTmfo^  "-^^  throw 

image  on  the  opposite  wall,  the  image  is  lar^e 

and  the  ima|e  is  small 

and  the  distance  between  the  candle  and  glass  is  ver^ 
much  increased.  ^  ^ 

f^^J'''''  f^^:^.  you   an   instrument  called  a 
Scioptr^c  Ball,  which  is  fastened  into  a  window  shutter 
in  a  room  from  which  all  light  is  banished  excep 
what  comes  m  through  this  glass.  ^ 

C.  Of  what  does  this  instrument  consist  ?  , 
.  ^  •  V?  ^  F^n^e  A  B  and  a  ball  of  wood  c  rf^ 
m  which  IS  a  glass  lens;  and  the  ball  cJ  ^ 
moves  eesily  m  the  frame  in  all  directions,  H  ^ 
so  that  the  view  of  any  surrounding  objects  H  O 
may  be  received  through  it. 

shu'tte??'  ^'^'^  ^""'^^         ^'^'^^  '"^^^ 

them  l^X^tt     '"^      \       'h^^  P"^P««^  ^ 
inere  are  little  brass  screws  belonging  to  it,  such  as 

hat  marked  5.  When  it  is  fixed  in  its  place  a 
screen  must  be  placed  at  a  proper  distance  from  ihe 
doors     Th-r-'  T      '"'^'^  objects  out  of 

ficial  eye        ^"^^^^^^^"^      sometimes  called  an  arti- 

R 


362 


OPTICS. 


C.  In  what  respects  is  it  like  the  eye  1 

T,  The  frame  has  been  compared  to  the  socket  in 
which  tlie  eye  moves,  and  the  wooden  ball  to  the 
whole  globe  of  the  eye ;  the  hole  in  the  ball  repre- 
sents the  pupil,  the  convex  lens  corresponds  to  the 
crystalline  humour,*  and  the  screen  to  the  retina. 

J.  The  ball  by  turning  in  all  directions  is  very  like 
the  eye,  for  without  moving  my  head  I  can  look  on 
all  sides,  and  upwards  and  downwards. 

T.  Well,  we  will  now  place  the  screen  properly, 
and  turn  the  ball  to  the  garden  : — Here  you  see  all 
the  objects  perfectly  expressed. 

/.  But  they  are  all  inverted. 

T,  That  is  the  great  defect  belonging  to  this  in- 
strument ;  but  1  will  tell  you  how  it  may  be  re- 
medied : — take  a  looking  glass  and  hold  it  before  you 
with  its  face  towards  the  picture  on  the  screen,  and 
inclining  a  little  downwards,  and  the  images  will 
appear  erect  in  the  glass,  and  even  brighter  than 
they  were  on  the  screen. 

C.  You  have  shewn  us  in  what  manner  the  rays  of 
light  are  refracted  by  convex  lenses,  when  those  rays 
are  parallel :  will  there  not  be  a  difference  if  the 
rays  converge  or  diverge  before  they  enter  the  lens  1 

T.  Certainly  :  if  rays  converge  before  they  enter  a 
convex  lens,  they  will  be  collected  at  a  point  nearer 
to  the  lens  than  the  focus  of  parallel  rays.  But  if 
they  diverge  before  they  enter  the  lens,  they  will  then 
be  collected  in  a  point  beyond  the  focus  of  parallel 
rays. 

There  are  concave  lenses  as  well  as  convex,  and 
the  refraction  which  takes  place  by  means  of  these 
differs  from  that  which  I  have  already  explained.  | 

C.  What  will  the  effect  of  refraction  be  when! 
parallel  rays  fall  upon  a  double  concave  lens  ?  1 

*  For  an  explanation  of  these  terms,  see  Coiiver.  XV.  1 
on  the  Eye.  I 


OF  CONCAVE  LENSES  g63 
T.  Suppose  the  parallel  rays,  a 
a,  h,  Cy  d,  &c.  pass  through  the 
lens  A  B,  they  will  diverge  after 
they  have  passed  through  the 
glass. 

J ,  Is  there  any  rule  for  ascer- 
taining the  degree  of  divergency  ?  Fig.  14. 

T.  Yes,  it  will  be  precisely  so 
much  as  if  tlie  rays  had  come  from  a  radiant  point  x, 
which  is  the  centre  of  the  concavity  of  the  glass. 

C.  Is  that  point  called  the  focus  ? 

T.  It  is  called  the  virtual  or  imaginary  focus. 
Thus  the  ray  a,  after  passing  through  the  glass  a  b 
will  go  on  in  the  direction  ^ /z,  as  if  it  had  come  from 
the  point  x,  and  no  glass  been  in  the  way  ;  the  ray  b 
w^ould  go  on  in  the  direction  m  n,  and  the  ray  e  in  the 
direction  r  s,  and  so  on.  The  ray  cx  in  the  centre 
suffers  no  refraction,  but  proceeds  precisely  as  if  no 
glass  had  been  in  the  way. 

/ .  Suppose  the  lens  had  been  concave  only  on  one 
side,  and  the  other  side  had  been  flat,  how  would  the 
rays  have  diverged  1 

T.  They  would  have  diverged  after  passing 
through  it,  as  if  they  had  come  from  a  radiant  point 
at  the  distance  of  a  whole  diameter  of  the  convexity 
of  the  lens. 

C.  There  is  then  a  great  similarity  in  the  refrac- 
tion of  the  convex  and  concave  lens. 

T.  True :  the  focus  of  a  double  convex  is  at  the 
distance  of  the  radius  of  convexity,  and  so  is  the 
imaginary  focus  of  the  double  concave;  and  the  focus 
of  the  plano-convex  is  at  the  distance  of  the  diameter 
of  the  convexity,  and  so  is  the  imaginary  focus  of  the 
plano-concave. 

You  will  find  that  images  formed  by  a  concave 
leiis,  or  those  formed  by  a  convex  lens,  where  the 
object  is  ivithin  its  principal  focus,  are  in  the  same 
position  with  the  objects  they  represent :— they  are 
also  imaginary,  for  the  refracted  rays  never  meet  at 
the  foci  whence  they  seem  to  diverge. 


364 


OPTICS. 


But  the  images  of  objects  placed  beyond  the  focus 
of  a  convex  lens  are  inverted,  and  i^eul,  for  the  re- 
fracted rays  do  meet  at  their  proper  foci. 

Remember,  convex  lenses  render  the  rays  which 
pass  through  them  convergent,  and  bring  them  to- 
gether into  a  focus.  Concave  lenses  render  the  rays 
transmitted  through  them  more  divergent. 


CONVERSATION  VIII. 

OF  THE  NATURE  AND  ADVANTAGES  OF  LIGHT  OF  THE 

SEPARATION  OF  THE  RAYS  OF  LIGHT  BY  MEANS  OF 
A  PRISM — AND  OF  COMPOUND  RAYS,  &C. 

T.  We  cannot  contemplate  the  nature  of  light 
without  being  struck  with  the  great  advantages  which 
we  enjoy  from  it.  Without  that  blessing  our  con- 
dition would  be  truly  deplorable. 

C.  I  well  remember  how  feelingly  Milton  de- 
scribes his  situation  after  he  lost  his  sight  • 

 With  the  year 

Seasons  retura  ;  but  not  to  me  returns 
Day,  or  the  sweet  approach  of  even  or  morn. 
Or  sight  of  vernal  bloom,  or  summer's  rose. 
Or  flocks,  or  herds,  or  human  face  divine  ; 
But  cloud  instead,  and  ever-during-  dark 
Surrounds  me,  from  the  cheerful  ways  of  men 
Cut  off,  and  for  the  book  of  knowledge  fair 
Presented  with  an  universal  blank 
Of  Nature's  works,  to  me  expunged  and  razed, 
And  wisdom,  at  one  entrance,  quite  shut  out. 

T.  Yet  his  situation  was  rendered  comfortable  by 
means  of  friends  and  relations,  who  all  possessed  the 
advantages  of  light.  But  if  our  world  were  deprived 
of  light,  what  pleasure  or  even  comfort  could  we  en- 
joy] How,"  says  a  good  writer,  **  could  we  provide 
ourselves  with  food,  and  the  other  necessaries  of  life? 
How  could  we  transact  the  least  business?  How 
could  we  correspond  with  each  other,  or  be  of  the 


THE  BLESSINGS  OF  LIGHT. 


305 


least  reciprocal  service,  without  light,  and  those  ad- 
mirable organs  of  the  body  which  the  Omnipotent 
Creator  has  adapted  to  the  perception  of  this  inesti- 
mable benefit  ?" 

J.  But  you  have  told  us  that  the  light  would  be 
of  comparatively  small  advantage  without  an  atmos- 
phere, 

T.  The  atmosphere  not  only  refracts  the  rays  of 
the  light  so  that  we  enjoy  longer  days  than  we  should 
without  it,  but  occasions  that  twilight  which  is  so 
beneficial  to  our  eyes,  for  without  it  the  appearance 
and  disappearance  of  the  sun  would  have  been  in- 
instantaneous  ;  and  in  every  twenty-four  hours  we 
should  have  experienced  a  sudden  transition  from  the 
brightest  sun-shine  to  the  most  profound  darkness,  and 
from  thick  darkness  to  a  blaze  of  light. 

C.  I  know  how  painful  that  would  be,  from  having 
slept  in  a  very  dark  room  and  having  suddenly  opened 
the  shutters  when  the  sun  was  shining  extremely  bright. 

T.  The  atmosphere  reflects  also  the  light  in  every 
direction,  and  if  there  were  no  atmosphere,  the  sun 
would  benefit  those  only  who  looked  towards  it,  and 
to  those  wliose  backs  were  turned  to  that  luminary  it 
would  all  be  darkness.  Ought  we  not,  therefore, 
gratefully  to  acknowledge  the  wisdom  and  goodness 
of  the  Creator,  who  has  adapted  these  things  to  the 
advantage  of  his  creatures  ? 

J.  I  saw  in  some  of  your  experiments  that  the  rays 
of  light  after  passing  through  the  glass  were  tinged 
with  diflferent  colours  ;  what  is  the  reason  of  this  ? 

2\  Formerly  light  was  supposed  to  be  a  simple 
and  uncompounded  body  ;  Sir  Isaac  Newton,  how- 
ever, discovered,  that  it  was  not  a  simple  substance, 
but  was  composed  of  several  parts,  each  of  which  has 
in  fact  a  different  degree  of  refrangibiiity. 

C.  How  is  that  shewn  ? 

T.  Let  the  room  be  darkened,  and  let  there  only 
be  a  very  small  hole  in  the  shutter  to  admit  the  sun's 
rays  ;  instead  of  a  lens  I  take  a  triangular  piece  or 


3C6 


OPTICS. 


glass,  called  a  prism ;  now,  as  in  this  there  is 
nothing  to  bring  the  rays  to  a  focus,  they  will,  in 
passing  through  it,  suffer  different  degrees  of  refrac- 
tion, and  be  separated  into  the  different  coloured  rays, 
which,  being  received  on  a  sheet  of  white  paper,  will 
exhibit  the  seven  following  colours,  red,  orange,  yellow, 
green,  blue,  indigo,  and  violet, 

J,  Here  are  all  the  colours  of  the  rainbow  :  the 
image  on  the  paper  is  a  sort  of  oblong. 

T.  That  oblong  image  is  usually  called  a  spectrum, 
and  if  it  be  divided  into  360  equal  parts,  the  red  will 
occupy  45  of  them,  the  orange  27,  the  yellow  48,  the 
green  and  the  blue  60  each,  the  indigo  40,  and  the 
violet  80. 

C.  The  shade  of  difference  in  some  of  these  colours 
seems  very  small  indeed. 

T.  You  are  not  the  only  person  who  has  made 
this  observation  ;  some  experimental  philosophers  say 
there  are  but  three  original  and  truly  distinct  colours, 
viz.  the  red,  yellow,  and  blue, 

C.  What  is  called  the  orange  is  surely  only  a  mix- 
ture of  the  red  and  yellow,  between  which  it  is 
situated. 

T.  In  like  manner  the  green  is  said  to  be  a  mixture 
of  the  yellow  and  blue,  and  the  violet  is  but  a  fainter 
tinge  of  the  indigo. 

J.  How  is  it  then  that  light,  which  consists  of 
several  colours,  is  usually  seen  as  white? 

T.  By  mixing  the  several  colours  in  due  proportion 
white  may  be  produced. 

J.  Do  you  mean  to  say  that  a  mixture  of  red, 
orange,  yellow,  green,  blue,  indigo,  and  violet,  in  any 
proportion,  will  produce  a  white  ] 

T,  If  you  divide  a  circular  surface  into  360  parts, 
and  then  paint  it  in  the  proportion  just  mentioned, 
that  is,  45  of  the  parts  red,  27  orange,  48  yellow,  ^ 
&c.  and  turn  it  round  with  great  velocity,  the  whole  ^ 
will  appear  of  a  dirty  white,  and  if  the  colours  were 
more  perfect  the  white  v/ould  be  so  too. 


OF  COLOURS. 


367 


J.  Was  it  then  owing  to  the  separation  of  the 
different  rays,  that  I  saw  the  rainbow  colours  about 
the  edges  of  the  image  made  with  the  lens? 

T.  It  was :  some  of  the  rays  were  scattered,  and 
not  brought  to  a  focus,  and  these  were  divided  in  the 
course  of  refraction.  And  I  may  tell  you  now,  though 
I  shall  not  explain  it  at  present,  that  the  rainbow  in 
the  heavens  is  caused  by  the  separation  of  the  rays  of 
light  mto  their  component  parts. 

C.  And  was  that  the  cause  of  the  colours  which  we 
saw  on  some  soap  bubbles  which  James  was  making 
with  a  tobacco-pipe  ? 

T.  It  was.  These  bubbles  are  merely  thin  bladders 
of  the  solution,  whose  thickness  continually  varying 
produces  the  variety  of  colours  which  they  exhibit. 

CONVERSATION  IX. 

OF  COLOURS. 

C.  After  what  you  said  yesterday,  I  am  at  a  loss  to 
know  the  cause  of  different  colours :  the  cloth  on  this 
table  is  green ;  that  of  which  my  coat  is  made  is  blue : 
what  makes  the  difference  in  these? 

T.  All  colours  are  supposed  to  exist  only  in  the 
light  of  luminous  bodies,  such  as  the  sun,  a  candle, 
&c.  and  that  light  falling  incessantly  upon  different 
bodies  is  separated  into  its  seven  primitive  colours, 
some  of  which  are  absorbed,  while  others  are  reflected. 

J.  Is  it  from  the  reflected  rays  that  we  judge  of  the 
colour  of  objects  ? 

T.  It  has  generally  been  thought  so;  thus  the  cloth 
on  the  table  absorbs  all  the  rays  but  the  green,  which 
it  reflects  to  the  eye :  but  your  coat  is  of  a  different 
texture,  and  absorbs  all  but  the  blue  rays. 

C.  Why  is  paper  white,  or  the  snow  7 

T.  The  whiteness  of  paper  is  occasioned  by  its  re- 
flecting the  greatest  part  of  all  the  rays  that  fall  upon 
it.  And  every  flake  of  snow,  being  an  assemblage  of 
frozen  globules  of  water  sticking  together,  reflects  and 


OPTICS. 


refracts  the  light  that  falls  upon  it  in  all  directions,  so 
as  to  mix  it  very  intimately,  and  produce  a  white  image 
on  the  eye. 

J.  Does  the  whiteness  of  the  sun's  light  arise  from 
a  mixture  of  all  the  primary  colours? 

T,  It  does,  as  may  be  easily  proved  by  an  experi- 
ment, for  if  any  of  the  seven  colours  be  intercepted  at 
the  lens,  the  image  in  a  great  measure  loses  its  white- 
ness. With  the  prism  I  will  divide  the  ray  into  its 
seven  colours  ;*  I  will  then  take  a  convex  lens  in  order 
to  re-unite  them  into  a  single  ray,  which  will  exhibit 
a  round  image  of  a  shining  white ;  but  if  only  fiv^e  or 
six  of  these  rays  be  taken  with  the  lens,  it  will  pro- 
duce a  dusky  white. 

C,  And  yet  to  this  white  colour  of  the  sun  we  are 
indebted  for  all  the  fine  colours  exhibited  in  nature. 

T.  Yes,  and  without  light  even  the  diamond  would 
lose  all  its  beauty. 

J.  The  diamond,  I  know,  owes  its  brilliancy  to  the 
power  of  reflecting  almost  all  the  rays  of  light  that 
fall  on  it :  but  are  vegetable  and  animal  tribes  equally 
indebted  to  it? 

T.  What  does  the  gardener  do  to  make  his  endive 
and  lettuces  white? 

C.  He  ties  them  up. 

T.  That  is,  he  shuts  out  the  light,  and  by  this  means 
they  become  blanched.  I  could  produce  you  a  thou  - 
sand instances  to  shew,  not  only  that  the  colour,  but 
even  the  existence,  of  vegetables  depends  upon  light. 
Close  wooded  trees  have  only  leaves  on  the  outside, 
such  is  the  cedar  in  the  garden.  Look  up  the  inside 
of  a  yew  tree,  and  you  will  see  that  the  inner  branches 
are  almost  oi  altogether  barren  of  leaves.  Gera- 
niums and  other  green-house  plants  turn  their  flowers 
to  the  light;  and  plants  in  general,  if  doomed  to  dark- 
ness, soon  sicken  and  die. 

J,  There  are  some  flowers,  the  petals  of  which  are, 

*  A  figure  will  be  given  on  this  subject  Avith  explana 
tions,  Conversation  XVI i I.  on  the  Rainbow. 


OF  COLOURS. 


in  different  parts,  of  different  colours  ;  how  do  you  ac- 
count for  this? 

T.  The  flower  of  the  heart's-ease  is  of  this  kind,  and 
if  examined  with  a  good  microscope  it  will  be  found  that 
the  texture  of  the  blue  and  yellow  parts  is  very  dif- 
ferent. The  texture  of  the  leaves  of  the  white  and  red 
rose  is  also  different.  Clouds  also,  which  are  so  vari- 
ous in  their  colours,  are  undoubtedly  more  or  less  dense, 
as  well  as  being  differently  placed  with  regard  to  the 
eye  of  the  spectator ;  but  they  all  depend  on  the 
light  of  the  sun  for  their  beauty. 

C.  Are  we  to  understand  that  all  colours  depend  on 
the  reflection  of  the  several  coloured  rays  of  light  1 

T.  This  seems  to  have  been  the  opinion  of  Sir  Isaac 
Newton;  but  he  concluded  from  various  experiments 
on  this  subject,  that  every  substance  in  nature,  provided 
it  be  reduced  to  a  proper  degree  of  thinness,  is  trans- 
parent. Many  transparent  media  reflect  one  colour, 
and  transmit  another:  gold-leaf  reflects  the  yellow, 
but  it  transmits  a  sort  of  green  colour  by  holding  it 
up  against  a  strong  light. 

Mr.  Delaval,  a  gentleman  who  a  few  years  since 
made  many  experiments  to  ascertain  how  colours  are 
produced,  undertakes  to  shew  that  they  are  exhibited 
by  transmitted  light  alone,  and  not  by  reflected  light. 

/.  I  do  not  see  how  that  can  be  the  case  with  bodies 
that  are  not  transparent. 

T.  He  infers,  from  his  experiments,  which  you  may 
hereafter  examine  for  yourselves,  that  the  original 
fibres  of  all  substances,  when  cleared  of  heterogeneous 
matter,  are  perfectly  white,  and  that  the  rays  of  light 
are  reflected  from  these  white  particles  through  the 
colouring  matter  with  which  they  are  covered,  and  that 
this  colouring  matter  serves  to  intercept  certain  rays 
in  their  passage  through  it,  while  a  free  passage  being 
left  to  others,  they  will  exhibit,  according  to  these  cir- 
cumstances, different  colours. — The  red  colour  of  the 
shells  of  lobsters  after  boiling,  he  says,  is  only  a  super- 
ficial covering  spread  over  the  white  calcareous  earth 
R  2 


370 


OPTICS. 


of  which  the  shells  are  composed,  and  maybe  removed 
by  scraping  or  filing.  Before  the  application  of  heat 
it  is  so  thick  as  to  appear  black,  being  too  thick  to  admit 
the  passage  of  light  to  the  shell  and  back  again.  The 
case  is  the  same  with  feathers,  which  owe  their  colours 
to  a  thin  layer  of  transparent  matter  on  a  white  giiDund. 

CONVERSATION  X. 

REFLECTED   LIGHT,  AND   PLAIN  MIRRORS. 

T.  We  come  now  to  treat  of  a  different  species  of 
glasses,  viz.  of  mirrors,  or,  as  they  are  sometimes 
called,  specula. 

J.  A  looking-glass  is  a  mirror,  is  it  not? 

T.  Mirrors  are  made  of  glass,  silvered  on  one  side; 
they  are  also  made  of  highly-polished  metal.  There 
are  three  kinds  of  mirrors,  the  plain,  the  convex,  and 
the  concave. 

C.  You  have  shewn  us  that,  in  a  looking-glass  or 
plain  mirror,  "The  angle  of  reflection  is  always  equal 
to  the  angle  of  incidence."* 

T.  This  rule  is  not  only  applicable  to  plain  mirrors, 
but  to  those  which  are  convex  and  concave  also,  as  I 
shall  shew  you  to-morrow.  But  I  wish  to  make  some 
observations  first  on  plain  mirrors.  In  the  first  place, 
if  you  wish  to  see  the  complete  image  of  yourself  in 
a  plain  mirror,  or  looking-glass,  it  must  be  half  as  long 
as  you  are  high. 

J.  I  should  have  imagined  the  glass  must  have  been 
as  long  as  I  am  high. 

T.  In  looking  at  your  image  in  the  glass,  does  it  not 
seem  to  be  as  far  behind  the  glass  as  you  stand  before 
it? 

/.  Yes :  and  if  I  move  forwards  or  backwards,  the 
image  behind  the  glass  seems  to  approach  or  recede. 
T.  Let  a  6  be  the  looking-glass,  and  a  the  spectator^ 

*  See  Conversation  II. 


OF  PLAIN  MIRRORS. 


371 


Fig.  15. 

standing  opposite  to  it.  The  ray  from  his  eye  will  be 
reflected  in  the  same  line  a  a,  but  the  ray  c  b,  flowing 
from  his  foot,  in  order  to  be  seen  at  the  eye,  must  be 
reflected  by  the  line  h  a. 

C.  So  it  will;  for  if  a?  6  be  a  line  perpendicular  to 
the  glass,  the  incident  angle  will  hQ  g  b  x,  equal  to  the 
reflected  angle  a  b  x. 

T.  And  therefore  the  foot  will  appear  behind  the 
glass  at  D  along  the  line  a  6  d,  because  that  is  the  line 
in  which  the  ray  last  approaches  the  eye. 

/.Is  that  part  of  the  glass  a  b  intercepted  by  the 
lines  A  B  and  a  d,  equal  exactly  to  half  the  length  b  d, 
or  AC? 

T.  It  is ;  Aa  b  and  a  b  d  may  be  supposed  to  form 
two  triangles,  the  sides  of  which  always  bear  a  fixed 
proportion  to  one  another;  and  if  a  b  is  double  of  a  a, 
as  in  this  case  it  is,  b  d  will  be  double  of  a  b,  or  at 
least  of  that  part  of  the  glass  intercepted  by  a  b  and 
a  d. 

C.  This  will  hold  true,  I  see,  stand  at  what  distance 
we  please  from  the  glass. 

T.  If  you  walk  towards  a  looking-glass  your  image 
will  approach,  but  with  a  double  velocity,  because  the 
two  motions  are  equal  and  contrary.  But  if  while  you 
stand  before  a  looking-glass,  your  brother  walk  up  to 
you  from  behind,  his  image  will  appear  to  you  to  move 
i&t  the  same  rate  as  he  walks,  but  to  him  the  velocity 
of  the  image  will  appear  to  be  double;  for  with  re- 
gard to  you,  there  will  be  but  one  motion,  but  with  re- 
gard to  him,  there  will  be  two  equal  and  contrary  ones. 

J.  If  I  look  at  the  reflection  of  a  candle  in  a  look- 
ing-glass, I  see  in  fact  two  images,  one  much  fainter 
than  the  other;  what  is  the  reason  of  this'? 


ST2 


OPTICS. 


r.  The  same  may  be  observed  of  any  object  that 
is  strongly  illuminated,  and  the  reason  of  the  double 
image  is,  that  a  part  of  the  rays  are  immediately  re- 
flected from  the  upper  surface  of  the  glass,  which  form 
the  faint  image,  while  the  greater  part  of  them  are  re- 
flected from  the  farther  surface,  or  silvering  part,  and 
form  the  vivid  image.  To  see  these  two  images  you 
must  stand  a  little  side- ways,  and  not  directly  before 
the  glass. 

C.  What  is  meant  by  the  expression  of  *'an  image 
being  formed  behind  a  reflector?" 

T.  It  is  intended  to  denote  that  the  reflected  rays 
come  to  the  eye  with  the  same  inclination  as  if  the  ob- 
ject itself  were  actually  behind  the  reflector.  If  you, 
standing  on  one  side  of  the  room,  see  the  image  of  your 
brother,  who  is  on  the  other  side,  in  the  looking-glass, 
the  image  seems  to  be  formed  behind  the  glass,  that  is, 
the  rays  come  to  your  eye  precisely  in  the  same  way 
as  they  would  if  your  brother  himself  stood  in  that 
place,  without  the  intervention  of  a  glass. 

J.  But  the  image  in  the  glass  is  not  so  bright  or 
vivid  as  the  object. 

T.  A  plain  mirror  is  in  theory  supposed  to  reflect  all 
the  light  which  falls  upon  it,  but  in  practice  nearly 
half  the  light  is  lost  on  account  of  the  inaccuracy  of 
tlie  polish,  &c. 

C.  Has  it  not  been  said,  that  Archimedes,  at  the 
siege  of  Syracuse,  burnt  the  ships  of  Marcellus  by  a 
machine  composed  of  mirrors'? 

T.  Yes :  but  we  have  no  certain  accounts  that  may 
be  implicitly  relied  on.  M.  Buflbn,  about  fifty  or 
sixty  years  ago,  burnt  a  plank  at  the  distance  of 
seventy  feet,  with  forty  plain  mirrors. 

./.  1  do  not  see  how  they  can  act  as  burning-glasses. 

T.  A  plain  mirror  reflects  the  light  and  heat  coming 
from  the  sun,  and  will  illuminate  and  heat  any  sub- 
stance on  which  they  are  thiown,  in  the  same  manner 
as  if  the  sun  shone  upon  it.  Two  mirrors  will  reflect 
on  it  a  double  quantity  of  heat ;  and  if  40  or  100 
mirrors  could  be  so  placed  as  to  reflect  from  each  the 


OF  CONCAVE  MIRRORS. 


373 


heat  coming  from  the  sun,  on  any  particular  substance, 
they  would  increase  the  heat  40  or  100  times. 


OF  CONCAVE  MIRRORS  THEIR  USES  HOW  THEY  ACT, 

J.  To  what  uses  are  concave  mirrors  applied? 
T.  They  are  chiefly  used  in  reflecting  telescopes : 
that  is,  in  telescopes  adapted  to  viewing  the  heavenly 
bodies.  And  as  you  like  to  look  at  Jupiter's  little 
moons  and  Saturn's  ring  through  my  telescope,  it  may 
be  worth  your  while  to  take  some  pains  to  know  by 
what  means  this  pleasure  is  afforded  you. 

T.  I  shall  not  object  to  any  attention  necessary  to 
comprehend  how  these  instruments  are  formed. 

T.  A  B  represents  a  concave 
mirror,  and  a  b,  c  d,  e  f,  three      A.#  3? 


the  mirror  a  b  in  all  its  parts. 

J.  Then  all  the  lines  drawn  from  c  to  the  glass  will 
be  equal  to  one  another,  as  c     c  d,  and  cf? 

T.  They  will :  and  there  is  another  property  be- 
longing to  them  J  they  are  all  perpendicular  to  the 
glass  in  the  parts  where  they  touch. 

C.  That  is,  c  b  and  0^' are  perpendicular  to  the  glas 
at  b  and  /,  as  well  as  c  c/  at  . 

T.  Yes,  they  are : — c  d  is  an  hicident  ray,  but  as  it 
passes  through  the  centre  of  concavity,  it  will  be  re- 
flected back  in  the  same  line  ;  that  is,  as  it  makes  no 
angle  of  incidence,  so  there  will  be  no  angle  of  reflec- 
tion :  a  6  is  an  incident  ray,  and  I  want  to  know  what 
will  be  the  direction  of  the  reflected  ray  ] 

C.  Since  c  6  is  perpendicular  to  the  glass  at  the 
angle  of  incidence  is  a'b  c\  and  as  the  angle  of  re- 


CONVEKSATION  XT 


parallel  rays  of  light  falling  upon 
it.  c  is  the  centre  of  concavity, 
that  is,  one  leg  of  your  com- 
passes being  placed  on  c,  and 
then  opening  them  to  the  length 
c  d,  and  the  other  leg  will  touch 


B 


Fig.  16. 


474 


OPTICS. 


fiect'on  is  always  equal  to  the  angle  of  incidence,  I 
must  make  another  angle,  as  c  b  m  equal  to  a  b  c,* 
and  then  the  line  b  m  is  that  in  which  the  incident  ray 
will  move  after  reflection, 

7'.  Can  you,  James,  tell  me  how  to  find  the  line  in 
which  the  incident  ray  e  /"will  move  after  reflection? 

J.  Yes:  I  will  make  the  angle  c /"  m  equal  to  c /*^, 
and  the  line  /  m  will  be  that  in  which  the  reflected 
ray  will  move  ;  therefore,  ef  is  reflected  to  the  same 
point  m  as  a  6  was. 

T.  If^  instead  of  two  incident  rays,  any  number  were 
drawn  parallel  to  c  d,  they  would  every  one  be  reflected 
to  the  same  point  m;  and  that  point,  which  is  called 
the  focus  of  parallel  rays,  is  distant  from  the  mirror 
equal  to  half  the  radius  c  d. 

J.  Then  we  may  easily  find  the  point  without  the 
trouble  of  drawing  the  angles,  merely  by  dividing  the 
radius  of  concavity  into  two  equal  parts. 

T.  You  may.  The  rays,  as  we  have  already  ob- 
served, which  proceed  from  any  point  of  a  celestial 
object,  may  be  esteemed  parallel  at  the  earth,  and 
therefore  the  image  of  that  point  will  be  formed  at  m. 

C.  Do  you  mean  that  all  the  rays  flowing  from  a 
point  of  a  star,  and  falling  upon  such  a  mirror,  will 
be  reflected  to  the  point  7n,  where  the  image  of  the  star 
will  appear  ? 

T.  I  do,  if  there  be  any  thing  at  the  point  m  to  re- 
ceive the  image. 

/.  Will  not  the  same  rule  hold  with  regard  to  ter- 
restrial objects  ? 

T.  No  :  for  the  rays  which  proceed  from  any  terres- 
trial object,  however  remote,  cannot  be  esteemed 
strictly  parallel ;  they  therefore  come  diverging,  and 
will  not  be  converged  to  a  single  poi)it,  at  the  distance 

*  To  make  an  angle  c  b  m,  equal  to  another  given  one, 
as  «  &  c.  From  h,  as  a  centre  with  any  radius  h  x,  de- 
scribe the  arc  x  o  which  will  cut  c  bin  z :  take  the  dis 
tance  .r  z  in  your  compasses,  and  set  off  with  it  z  o,  and 
then  draw  the  line  bom,  and  the  angle  w  6  c  is  equal 
to  the  angle  a  b  c. 


OF  CONCAVB  MIRRORS. 
of  half  the  radius  of  the  mirror's  concavity  from  thp 
-LTdlZce"/  "".V"  -'--^ atTli  5: 

C   Can  vn,    ^T-""!-"";" ^         ''^If  the  radius. 

U  Can  you  explam  this  by  a  figure? 

■( .  I  will  endeavour  to  do  so.    Let  a  b  be  a  con- 


Kg.  17. 

part  ofwMc'h'°'^  y  ^"^""^^  from  every 

rairror.  that  is,  from  the  point  m  ravs  will  flow  tn 
every  point  of  the  mirror,  and  so  they  will  i^^m  E  ai  d 
st  whiriyr'"'  '^'^^  extremities     L^t  us 

part  of  fhp'''!/'"       "if' P™"^^''       "'to  different 

r  V  ^^^J^''^^:}'^  '-eflected  to  a  single  points 
point.  T   'i  f ^         ^""^  the  difficulty  is  to  find  that 

the  glat;.  '  ""^  ^    '■^'^  of  concavity  of 

dicuiar^to'tJ  Y'^^ZV'  be  perpen- 

now  g[vfn  ani  ftt  t       P?'"'/-'  -  ^ 

r  ^7  j'        '    the  angle  of  incidence. 

did  before.^""  ^l"^'!  to  it,  as  you 

tend'  If  7       •  ^  ^  ^qtial  to  M  a  c,  and  ex- 

tend  the  line  A  X  to  any  length  you  please. 
M  c  and^h^       '  "  "  '"^''^  ^"h  the  ray 

of  incMe^ce.^''^'"'""'"  "    "'^'^'^    ^"°*«r  ^"g"^ 

to  k  Ind 'Jh^f '  °f  '•'^''^^tion  c  c  .  equal 

1  X  'nTl  ^""^  P''°'l"^^<i  -^uts  the  line 

A  ^  m  a  particular  point,  which  I  will  call  m. 


S76 


OPTICS. 


T.  Draw  now  the  perpendicular  c  b,  and  you  have, 
with  it  and  the  ray  m  b,  the  angle  of  incidence  m  b  c : 
make  another  angle  equal  to  it,  as  its  angle  of  reflection. 

J.  There  it  is,  c  b  u,  and  1  find  the  line  b  u  meets 
the  other  lines  at  the  point  m. 

T,  Then  m  is  the  point  in  which  all  the  reflected 
rays  of  m  will  converge ;  of  course  the  image  of  the 
extremity  m  of  the  arrow  e  m  will  be  formed  at  m. 
Now  the  same  might  be  shewn  of  every  other  part  of 
the  object  m  e,  the  image  of  which  will  be  represented 
by  e  w,  which  you  see  is  at  a  greater  distance  from  the 
glass  than  half  c  c,  or  radius. 

C.  The  image  is  inverted  also,  and  less  than  the 
object ;  which,  I  suppose,  will  always  be  the  case  in 
similar  circumstances. 


CONVERSATION  XII. 

OF  CONCAVE   MIRRORS,  AND  EXPERIMENTS  ON  THEM. 

T.  If  you  understand  what  we  conversed  on  yester- 
day, and  what  you  have  yourselves  done,  you  will 
easily  see  how  the  image  is  formed  by  the  large  con- 
cave mirror  of  the  reflecting  telescope  when  we  come 
to  examine  the  construction  of  that  instrument : — In 
a  concave  mirror  the  image  is  less  than  the  object, 
when  the  object  is  more  remote  from  the  mirror  than 
c,  the  centre  of  concavity,  and  in  that  case  the  image 
is  between  the  object  and  mirror. 

J.  Suppose  the  object  be  placed  in  the  centre  c? 

T.  Then  the  image  and  object  will  coincide  : — and 
if  the  object  is  placed  nearer  to  the  glass  than  the 
centre  c,  then  the  image  will  be  more  remote,  and  big- 
ger than  the  object. 

C.  1  should  like  to  see  this  illustrated  by  an  experi- 
ment. 

T.  Well,  here  is  a  large  concave  mirror:  place 
yourself  before  it,  beyond  the  centre  of  the  concavity; 
and  with  a  little  care  in  adjusting  your  position,  you 
will  see  an  inverted  image  of  yourself  in  the  air  be- 


OF  CONCAVE  MIRRORS.  37T 
tweeii  you  and  the  mirror,  and  of  a  less  size  than  you 
are.  When  you  see  the  image,  extend  your  hand 
gently  towards  the  glass,  and  the  hand  of  the  image 
will  advance  to  meet  it,  till  they  both  meet  in  the 
centre  of  the  glass's  concavity.  If  you  carry  your  hand 
still  farther,  the  hand  of  the  image  will  pass  by  it,  and 
come  between  it  and  the  body  :  now  move  your  hand 
to  either  side,  and  the  image  of  it  will  move  towards 
the  other. 

J.  Is  there  any  rule  for  finding  the  distance  at  which 
the  image  of  an  object  is  formed  from  the  mirror] 

T.  If  you  know  the  radius  of  the  mirror's  concavity, 
and  also  the  distance  of  the  object  from  the  glass, — 

Multiply  the  distance  and  radius  together,  and  di- 
vide the  product  by  double  the  distance  less  by  the 
radius,  and  the  quotient  is  the  distance  required." 

Tell  me  at  what  distance  the  image  of  an  object 
will  be,  suppose  the  radius  of  the  concavity  of  the 
mirror  be  12  inches,  and  the  object  be  at  18  inches 
from  it. 

J.  I  multiply  18  by  12,  which  gives  216;  this  I 
divide  by  double  18,  or  36,  less  by  12,  that  is  24;  but 
216  divided  by  24  gives  9,  which  is  the  number  of 
inches  required. 

T.  You  may  vary  this  example  in  order  to  impress 
the  rule  on  your  memory ;  and  I  will  shew  you 
another  experiment.  I  take  this  bottle  partly  full  of 
water,  and  corked,  and  place  it  opposite  the  concave 
mirror,  and  beyond  the  focus,  that  it  may  appear  to 
be  reversed :  now  stand  a  little  farther  distant  than  the 
bottle, 'eind  you  will  see  the  bottle  inverted  in  the  air, 
and  the  water  which  is  in  the  lower  part  of  the  bottle 
will  appear  to  be  in  the  upper. — I  will  invert  the  bottle 
and  uncork  it,  and  whilst  the  water  is  running  out  the 
image  will  appear  to  be  filling,  but  when  the  bottle  is 
empty  the  illusion  is  at  an  end. 

C.  Are  concave  mirrors  ever  used  as  burning- 
glasses  ] 

T.  Since  it  is  the  property  of  these  mirrors  to  cause 
parallel  rays  to  converge  to  a  focus,  and  since  the  rays 


378 


OPTICS. 


of  the  sun  are  considered  as  parallel,  tliey  are  very 
useful  as  burning-glasses,  and  the  principal  focus 
is  the  burning  point. 

J,  Is  the  image  formed  by  a  concave  mirror  always 
before  it  1 

T.  In  all  cases,  except  when  the  object  is  nearer 
to  the  mirror  than  the  principal  focus. 

C.  Is  the  image  then  behind  the  mirror  ? 
'  T,  It  is  ;  and  farther  behind 
the  mirror  than  the  object  is  a 


course  the  image  is  farther 
behind  the  glass  than  the  object  is  before  it. 

/.  What  would  be  the  effect  if,  instead  of  an  opaque 
object  xz,  a  luminous  one,  as  a  candle,  were  placed 
in  the  focus  of  a  concave  mirror  1 

T.  It  would  strongly  illuminate  a  space  of  the 
same  dimension  as  the  mirror  to  a  great  distance  ;  and 
if  the  candle  were  still  nearer  the  mirror  than  the 
focus,  its  rays  will  enlighten  a  larger  space.  Hence 
you  may  understand  the  construction  of  many  of  the 
lamps  which  are  now  to  be  seen  in  many  parts  of 
London,  and  which  are  undoubtedly  a  great  improve- 
ment in  lighting  the  streets.  Similar  principles  are 
frequently  employed  in  the  construction  of  reflectors 
for  lighthouses,  as  well  as  for  coach  and  other  lamps. 


before  it.  Let  ac  be  a  mirror, 
and  X  z  the  object  between  the 
centre  k  of  the  glass  and  the 
glass  itself ;  and  the  image  xi/  s 
will  be  behind  the  glass,  erect, 
curved,  and  magnified,  and  of 


Fig.  18. 


CONVERSATION  XIII. 


OF  CONCAVE  AND  CONVEX  MIRRORS. 


T,  We  shall  devote  another  morning  or  two  to  the 
subject  of  reflection  from  mirrors  of  different  kinds. 
C.  You  have  not  said  any  thing  about  coniei  mir- 


OF  CONVEX  MIRRORS.  379 
rors,  and  yet  they  are  now  very  much  in  fashion  in 
handsome  drawing-rooms :  I  remember  seeing  one 
when  I  was  at  uncle's  at  Bristol,  in  which  the  image 
was  very  much  less  than  the  object 

r.  A  convex  mirror  is  an  ornamental  piece  of  fur- 
niture, especially  if  it  can  be  placed  before  a  window, 
either  with  a  good  prospect,  or  where  there  are  a 
number  of  persons  passing  and  repassing  in  their 
different  employments.  The  images  reflected  from 
these  are  smaller  than  the  objects,  erect,  and  behind 
the  surface,  therefore  a  landscape  or  a  busy  scene  de- 
lineated on  one  of  them,  is  always  a  beautiful  object 
to  the  eye.  For  the  same  reason,  a  glass  of  this  kind 
in  a  room  in  which  large  assemblies  meet,  forms  an 
extremely  interesting  picture.  You  may  easily  con- 
ceive how  the  convex  mirror  diminishes  objects,  or 
the  images  of  objects,  by  considering  in  what  manner 
they  are  magnified  by  the  concave  mirror.  If  xyz 
(Fig.  18.)  were  an  object  before  a  convex  mirror  ac, 
the  image  by  reflection  would  be  xz. 

J.  Would  it  not  appear  curved  ? 

T.  Certainly  :  for  if  the  object  be  a  right  line,  or  a 
plain  surface,  its  image  must  be  curved,  because  the 
dififerent  points  of  the  object  are  not  equally  distant 
from  the  reflector.  In  fact,  the  images  formed  by 
convex  mirrors,  if  accurately  compared  with  the  ob- 
jects are  never  exactly  of  the  true  shape. 

C.  I  do  not  quite  comprehend  in  what  manner  re- 
flection takes  place  at  a  convex  mirror. 

T,  1  will  endeavour  by  a 
figure  to  make  it  plain  :  c  d  g/-, 
represents  a  convex  mirror  "^--^ 
standing  at  the  end  of  a 
room,  before  which  the  arrow  P'"'^ 
A  p  is  placed  on  one  side,  or 
obliquely;  where  must  the  Fig.  19. 

spectator  stand  to  see  the 
reflected  image  ? 

C.  On  the  other  side  of  the  room. 

r.  The  eye  e  will  represent  that  situation  : — the 


380  OPTICS, 
rays  from  tlie  external  parts  of  the  arrow  a  and  p  flow 
convergingly  along  a  a  and  p6,  and  if  no  glass  were 
in  the  way  tliey  would  meet  at  p  ;  but  the  glass  re- 
flects the  ray  a  a  along  a  e,  and  the  ray  vb  along  6e  ; 
and  as  we  always  transfer  the  image  of  an  object  in 
that  direction  in  which  the  rays  approach  the  eye,  we 
see  the  image  of  a  along  the  line  Ea  beliind  the  glass, 
and  the  image  of  p  along  e6,  and,  of  course,  the 
image  of  the  whole  arrow  is  at  s. 

By  means  of  a  similar  diagram  I  will  shew  you 
more  clearly  the  princi- 


ple of  the  concave  mir- 
ror. Suppose  an  object 
e  beyond  the  focus  f, 
and  the  spectator  to 
stand  at  s,  the  rays  e  b 


and  ed   are  reflected,  Fig,  20. 

and  where  they  meet  in 
E  the  spectator  will  see  the  image. 

J.  That  is  between  himself  and  the  object 
T.  He  must,  however,  be  far  enough  from  it  to 
receive  the  rays  after  they  have  diverged  from  e,  be- 
cause every  enlightened  point  of  an  object  becomes 
visible  only  by  means  of  a  cone  of  diverging  rays 
from  it,  and  we  cease  to  see  it  if  the  rays  become 
parallel  or  converging. 

C.  Is  the  image  inverted  1 

T.  Certainly,  because  the  rays  have  crossed  be- 
fore they  reach  the  eye. 

You  may  see  this  subject  in  another  point  of  view : 


Fig.  21. 


let  xy  be  a  concave  mirror,  and  o  the  centre  of  con- 
cavity :  divide  oa  equally  in  f,  and  take  the  half,  the 


OF  THE  CONCAVE  MIRROR.  381 
third,  the  fourth,  &c.  of  fo,  and  mark  these  divisions 
h  J»  h  Let  A  o  be  extended,  and  parts  be  taken 
in  it  equal  to  f  o,  at  2,  3,  4,  &c.  Now  if  any  of  the 
points  1,  2,  3,  4,  &c.  be  the  focus  of  incident  rays, 
tne  corresponding  points  1,  |,  J,  i,  &c.  in  of  will  be 
the  focus  of  the  reflected  rays,  and  vice  versa. 

J,  Do  you  mean  by  that,  if  incident  rays  be  at  |, 

i»  or  |,  the  reflected  rays  will  be  at  2,  3,  4  ? 

T,  I  do  :  place  a  candle  at  2,  and  an  inverted 
image  will  be  seen  at  J  :  now  place  it  at  4,  and  it  will 
also  move  back  to  i  :  these  images  may  be  taken  on 
paper  held  in  those  respective  places. 

C.  I  see  the  farther  you  proceed  one  way  with  the 
candle,  the  nearer  its  inverted  image  comes  to  the 
point  f. 

T.  True  :  and  it  never  gets  beyond  it,  for  that  is 
the  focus  of  parallel  rays  after  reflection,  or  of  rays 
that  come  from  an  infinite  distance. 

J.  Suppose  the  candle  were  at  o  ? 

T.  Then  the  object  and  image  will  coincide  :  and 
as  the  image  of  an  object  between  f  and  a  concave 
speculum  is  on  the  other  side  of  the  speculum, 
this  experiment  of  the  candle  and  paper  cannot  be 
made. 

I  will  now  just  mention  an  experiment  that  we 
may  hereafter  make.  At  one  end  of  an  oblong  box, 
about  two  feet  long,  and  fifteen  inches  wide,  is  to  be 
placed  a  concave  mirror  ;  near  the  upper  part  of  the 
opposite  end  a  hole  is  made,  and  about  the  middle  of 
the  box  is  placed  a  hollow  frame  of  pasteboard  that 
confines  the  view  of  the  mirror.  The  top  of  the  box 
next  the  end  in  which  the  hole  is  made  is  covered 
with  a  glass,  but  the  other  half  is  darkened.  Under 
the  hole  are  placed  in  succession  different  pictures, 
properly  painted,  which  are  thrown  into  perspective 
by  the  mirror,  and  produce  a  beautiful  appearance. 


I 


382 


OPTICS. 


CONVERSATION  XIV. 


OF  CONVEX  REFLECTION  OF  OPTICAL  DELUSIONS  OP 

ANAMORPHOSES. 

C.  You  cannot,  I  see,  make  the  same  experiment 
with  the  candle  and  a  convex  mirror,  that  you  made 
yesterday  with  the  concave  one. 

T.  Certainly,  because  the  image  is  formed  behind 
the  glass  ;  but  it  may,  perhaps,  be  worth  our  while  to 
consider  how  the  effect  is  produced  in  a  mirror  of  this 


kind.  Let  a  b  represent  a  convex  mirror,  and  a/ be 
half  the  radius  of  convexity,  and  take  a  f,  fo,  o  e, 
&c.  each  equal  a/.  If  incident  rays  flow  from  2, 
the  reflected  rays  will  appear  to  come  from  behind 
the  glass  at  ~. 

J,  Do  you  mean,  if  a  candle  be  placed  at  2,  the 
image  of  it  will  appear  to  be  formed  at  |  beliind  the 
glass  1 

T,  I  do  :  and  if  that  or  any  other  object  be  car- 
ried to  3,  4,  &c.  the  image  will  also  go  backward  to 

C.  Then,  as  a  person  walks  towards  a  convex  sphe- 
rical reflector,  the  image  appears 'to  walk  towards 
him,  constantly  increasing  in  magnitude  till  they 
touch  each  other  at  the  surface. 

T.  You  will  observe  that  the  image,  however  dis- 
tant the  object,  is  never  farther  off  than  at/ ;  that  is 
the  imaginary  focus  of  parallel  rays. 

/.  The  difference  then  between  convex  and  coa-i 
cave  reflectors  is,  that  the  point/ in  theforiner  is  beJ 


Fig.  22. 


OF  CONVEX  MIRRORS.  383 

hind  the  glass,  and  in  the  latter  it  is  before  the  Plass 
as  F,  o  ^ 

r  Just  so:  from  the  property  of  diminishing  ob- 
ects  spherical  reflectors  are  not  only  pleasing  orna- 
ments for  our  best  rooms,  but  are  much  used  by  aU 
overs  of  picturesque  scenery.  "  Small  convex  reflec- 
ttvell''^'  ">^de  for  the  use  of 

1  A  i     f'  ^^^"^  •'y  stretching  the  eye 

to  Alps  towering  on  Alps,  can  by  their  mirror  brin' 
ti  e  e  subhme  objects  into  a  narrow  compass,  and 
gratify  the  sight  by  pictures  which  the  art  of  man  in 
vain  attempts  to  imitate."  * 

Concave  mirrors  have  been  used  for  many  other 
and  different  purposes,  for  by  tnem,  with  a  little  in- 
genuity, a  thousand  illusions  may  be  practised  on  the 
Ignorant  and  credulous.  f        cu  on  me 

in  RoL^ir^^'  P'?"  exhibition 
in  Bond  Street,  which  you  said  depended  on  a  con- 
cave mirror  ;  I  was  desired  to  look  into  the  glass  I 
did  so,  and  started  back,  for  I  thought  the  point  of  a 

mot  h^f  /,  ''"^  '"'PP'^  >  then  I  slw  a 
most  beautiful  nosegay,  which  I  wished  to  grasp,  but 
It  vanished  in  an  instant  *  ^ 

T.  1  will  explain  how  these  deceptions  are  ma- 


Fig.  23. 

naged  :  let  e  p  be  a  concave  mirror,  10  or  12  inches  in 
diameter,  placed  in  one  room  ;  ^  b  the  wain  cot  that 
eparates  the  spectator  from  it,  but  in  this  there  is  a 
square  or  circular  opening  which  faces  the  mirror  ex- 

•  See  Economy  of  Nature,  Vol.  I.,  p.  26,  2d  Edition. 


384 


OPTICS. 


actly.  A  nosegay,  for  instance,  is  inverted  at  c,  which 
must  be  strongly  illuminated  by  means  of  an  Argand's 
lamp  ;  but  no  direct  light  from  the  lamp  is  to  fall  on 
the  mirror.  Now  a  person  standing  at  g  will  see  an 
image  of  the  nosegay  at  d. 

J.  What  is  to  make  it  vanish  1 

T.  In  exhibitions  of  this  kind  there  is  always  a  per- 
son behind  the  wainscot  in  league  with  the  man  that 
attends  the  spectator,  who  removes  the  real  nosegay 
upon  some  hint  understood  between  them. 

C.  Was  it  then  upon  the  man  behind  the  scene 
that  the  approaching  sword,  and  the  advancing  death's 
head,  &c.  depended'' 

T.  It  was  :  and  persons  have  undertaken  to  exhi- 
bit the  ghosts  of  the  dead  by  contrivances  of  this 
kind  ;  for  if  a  drawing  of  the  deceased  be  placed  in- 
stead of  the  nosegay,  it  may  be  done.  But  such  ex- 
hibitions are  not  to  be  recommended,  and  indeed 
ought  never  to  be  practised,  without  explaining  the 
whole  process  to  the  astonished  spectator  afterwaids. 

If  a  large  concave  mirror  be  placed  before  a 
blazing  fire,  so  as  to  reflect  the  image  of  the  fire  on  the 
flap  of  a  bright  mahogany  table,  a  spectator  suddenly 
introduced  in  the  room  will  suppose  the  fire  to  be  on 
the  table. 

If  two  large  concave  -^^x. 

mirrors  a  and  b  be  placed   1> 

opposite  each  other  at  the      |V  ^xl 
distance  of  several  feet,  Y\o-.  24. 

and  red  hot  charcoal  be  ° 
put  in  the  focus  d,  and  some  gunpowder  in  the  other 
focus  c,  it  will  presently  take  fire.  The  use  of  a  pair 
of  bellows  may  be  necessary  to  make  the  charcoal 
burn  strongly. — This  experiment  may  be  varied  by 
placing  a  thermometer  in  one  focus,  and  lighted  char- 
coal in  the  other,  and  it  will  be  seen  that  the  quick- 
silver in  the  thermometer  will  rise  as  the  fire  increases, 
though  another  thermometer  at  the  same  distance 
from  the  fire,  but  not  in  the  focus  of  the  glass,  will 
not  be  aflfected  by  it. 


OF  THiS  EYE.  gg. 

se™  to  astonish  those  who  a^etS 


CONVERSATION  XV. 

OF  THE  DIFFERENT  PABTS  OP  THE  EYE. 

:xs^t:irtr---^^^^^^^ 

^.  The  eye,  when  taken  from  the  socket,  is^  of  a 


Fig.  25. 


386 


OPTICS. 


globular  form,  and  it  is  composed  of  three  coats  or 
skins,  and  three  other  substances  called  humours. 
The  first  figure  represents  the  section  of  an  eye,  that 
is,  an  eye  cut  down  the  middle :  and  Fig.  26.  the 
front  view  of  an  eye  as  it  appears  in  the  head. 

C.  Have  these  coats  and  humours  aU  different 
names  ? 

T.  Yes :  the  external  coat,  which  is  represented 
by  the  outer  circle  A  BCD  E,  is  called  the  sclerotica: 
the  front  part  of  this,  namely,  cxb,  is  perfectly  tran- 
sparent, and  is  called  the  cornea  j  beyond  this,  to- 
wards B  and  E,  it  is  white,  and  called  the  white  of  the 
eye.  The  next  coat,  which  is  represented  by  the 
second  circle,  is  called  the  choroides, 

J.  This  circle  does  not  go  all  round. 

T,  No  :  the  vacant  space  ab  is  that  which  we  call 
the  pupil,  and  through  this  alone  the  light  is  allowed 
to  enter  the  eye. 

C.  What  do  you  call  that  part  which  is  of  a  beau- 
tiful blue  in  some  persons,  as  in  cousin  Lydia  j  and 
in  others  brown,  or  almost  black  ? 

T.  That,  as  ac,  be,  is  part  of  the  choroides,  and  i 
called  the  iris. 

C.  The  iris  is  sometimes  much  larger  than  it  is  at 
another. 

T»  It  is  composed  of  a  sort  of  net-work,  which  con 
tracts  or  expands  according  to  the  force  of  light  i 
which  it  is  placed.  Let  James  stand  in  a  dark  cor 
ner  for  two  or  three  minutes  : — now  look  at  his  eyes. 

C.  The  iris  of  each  is  very  smalt,  and  the  pupil 
large. 

T.  Now  let  him  look,  steadily,  pretty  close  to  thei 
candle.  j 

C.  The  iris  is  considerably  enlarged,  and  the  pupili 
of  the  eye  is  but  a  small  point  in  comparison  of  v.  haa 
it  was  before.  ^ 

T,  Did  you  never  feel  uneasy  after  sitting  some 
time  in  the  dark,  when  candles  were  suddenly 
brought  into  the  room  1  s 

/.  Yes:  1  lemember  last  Friday  evening  we  hadj 


OP  THE  HUMOURS  OF  THE  EYE.  3B7 
b3ea  fitting  half  an  hour  almost  in  the  dark  at  Mr 
onpTfThT^'.''  candles  were  introduced  every 

one  ot  the  company  complamed  of  the  pain  which  the 
suddea  light  occasioned.  ^ 

fr  J;»i^^  ''"'"^  ?  ■^^'-'^  the  iris  was  con- 

tacted very  much;  of  course,  the  pupil  being  laZ 
more  hght  was  admitted  than  it  could  well  befr  afd 
he  efore,  t.U  hme  was  allowed  for  the  iris  to  adfus 
Itself,  the  uneasmess  would  be  felt 

^^^'^  'f"^  "'"d  "coat,  which,  from 

in  ;l  .  f"^-  '^"^  net-work,  which  serves 

efr,  r    '  f\  ""I-^''  P^duced  by  The 

refraction  of  the  different  humours  of  the  eye  and 
pamted,  as  it  were,  on  the  surface.  ^ 
ino^'tht''''  ""^  ''f  intended  forrefract- 

iefses  '  '°  '''^  ^^""^  manner  as  glass 

nl'f  u^^^       ^""^  '''^y  ^'•^  ""e  «(r««„s,  the 

"  Vrta«,«.  and  the  ayu.ow  humours.    The  vitreous 
humour  fills  up  all  the  space  zz,  at  the  back  of  the 
cje;  ,t  IS  nearly  of  the  substance  of  melted  glass 
Idou2  '^^«Pr«^ented  by  df,  in  the  shape  of 

a  double  convex  lens:  and  the  aqueous,  or  waterJ 
humour,  fills  up  all  that  part  of  the  eye  betwein  the 
V^llT  ^°<1      cornea  c  x 

rep'^esm  f  "  eye 

irt,/^  ^-^  'l^*''^  optic  nerve,  which  serves  to  convey  to 
the  brain  the  sensations  produced  on  the  retina.  ^ 
!    y.  Does  the  retina  extend  to  the  brain' 

^.  It  does:  and  we  shall,  when  we  meet  next 
endeavour  to  explain  the  office  of  these  humours  fn 
affecting  vision.  In  the  mean  time,  I  woul"reques" 
you  to  consider  again  what  I  have  told  you  of  the 
ItXCVasVtr^        — -'-h-a'ml 

;|   ^.  1  intended  to  have  reserved  this  to  another 


388 


OPTICS. 


opportunity  :  but  I  may  now  say,  that  the  eye-brows 
defend  the  eye  from  too  strong  a  light ;  and  they  pre- 
vent the  eyes  from  injuries  by  the  sliding  of  substances 
down  the  forehead  into  them. 

The  eye-lids  act  like  curtains  to  cover  and  protect 
the  eyes  during  sleep  ;  when  we  are  awake,  they 
diffuse  a  fluid  over  the  eye,  which  keeps  it  clean  and 
well  adapted  for  transmitting  the  rays  of  light. 

The  eye-lashes,  in  a  thousand  instances,  guard  the 
eye  from  danger,  and  protect  it  from  floating  dust, 
with  which  the  atmosphere  abounds. 

CONVERSATION  XVI. 

OF  THE  EYE,  AND  THE  MANNER  OF  VISION. 

C.  I  do  not  understand  what  you  meant,  when  you 
said  the  optic  nerve  served  to  convey  to  the  brain  the 
sensations  produced  on  the  retina. 

T,  Nor  do  I  pretend  to  tell  you  in  what  manner 
the  image  of  any  object  painted  on  the  retina  of  the 
eye  is  calculated  to  convey  to  the  mind  an  idea  of 
that  object :  but  I  wish  to  shew  you,  that  the  images 
of  the  various  objects  which  you  see  are  painted  on 
the  retina.  Here  is  a  bullock^s  eye,  from  the  back 
part  of  which  I  cut  away  the  three  coats,  but  so  as  to 
leave  the  vitreous  humour  perfect :  I  will  now  put 
against  the  vitreous  humour  a  piece  of  white  paper, 
and  hold  the  eye  towards  the  window;  what  do  you 
see  ? 

/.  The  figure  of  the  window  is  drawn  upon  the 
paper  ;  but  it  is  inverted. 

T*.  Open  the  window,  and  you  will  see  the  trees  in 
the  garden  drawn  upon  it  in  the  same  inverted  state, 
or  any  other  bright  object  that  is  presented  to  it. 

C.  Does  the  paper  in  this  instance  represent  the 
innermost  coat,  called  the  retina  ? 

T,  It  does,  and  I  have  made  use  of  paper  because 
it  is  easily  seen  through,  whereas  the  retina  is  opaque  ; 
transpaiency  would  be  of  no  advantage  to  it.  The 


IMAGES  ON  THE  RETINA.  389 
retina,  by  means  of  the  optic  nerve,  is  conveyed  to 
the  brain,  or,  in  other  w^ords,  the  optic  nerve  is  an 
extension  of  the  retina. 

J.  And  does  it  carry  the  news  of  every  object  that 
is  painted  on  the  retina? 

T.  So  it  should  seem  j  for  w^e  have  an  idea  of 
whatever  is  drawn  upon  it.  I  direct  my  eyes  to  you, 
and  the  image  of  your  person  is  painted  on  the  retina 
of  my  eye,  and  I  say  I  see  you.    So  of  any  thing  else. 

C.  You  said  the  rays  of  light  proceeding  from  ex- 
ternal objects  were  refracted  in  passing  through  the 
different  humours  of  the  eye. 

T.  They  are,  and  converged  to  a  point,  or  there 
would  be  no  distinct  picture  drawn  on  the  retina, 
and  of  course  no  distinct  idea  conveyed  to  the  mind. 
I  will  shew  you  what  I  mean  by  a  figur.e,  taking  an 
arrow  again  as  an  illustration. 


Fig.  27. 

As  every  point  of  an  object  a  b  c  sends  out  rays  in 
all  directions,  some  rays  from  each  point  on  the  side 
next  the  eye,  will  fall  upon  the  cornea  between  x  y, 
and  by  passing  through  the  humours  of  the  eye,  they 
will  be  converged  and  brought  to  as  many  points  on 
the  retina,  and  will  form  on  it  a  distinct  inverted 
picture  c    a  of  the  object. 

J.  This  is  done  in  the  same  manner  as  you  shewed 
us  by  means  of  a  double  convex  lens. 

T.  All  three  of  the  humours  have  some  effect  in 
refracting  the  rays  of  light,  but  the  crystalline  is  the 
most  powerful,  and  that  is  a  complete  double  convex 
lens  :  and  you  see  the  rays  from  a  are  brought  to  a 


OPTICS. 


point  at  a  :  those  from  b  will  be  converged  at  h,  and 
those  from  c  at  c ;  and,  of  course,  the  intermediate 
ones  between  a  and  b,  and  b  and  c,  will  be  formed 
between  a  and  6,  and  b  and  c.  Hence  the  object  be- 
comes visible  by  means  of  the  image  of  it  being  drawn 
on  the  retina. 

C.  Since  the  image  is  inverted  on  the  retina,  how 
is  it  that  we  see  things  in  the  proper  position  ? 

T.  This  is  a  proper  question,  but  one  that  is  not 
very  readily  answered.  It  is  well  known  that  the 
sense  of  touch  or  feeling  very  much  assists  the  sense 
of  sight ;  some  paintings  are  so  exquisitely  finished, 
and  so  much  resemble  sculpture,  that  the  eye  is  com- 
pletely deceived ;  we  then  naturally  extend  the  hand 
to  aid  the  sense  of  seeing.  Children,  who  have  to 
learn  the  use  of  all  their  senses,  make  use  of  their  hands 
in  every  thing ;  they  see  nothing  which  they  do  not 
wish  to  handle,  and,  therefore,  it  is  not  improbable, 
that  by  the  sense  of  the  touch,  they  learn,  unawares, 
to  rectify  that  of  seeing.  The  image  of  a  chair,  or 
table,  or  other  object,  is  painted  in  an  inverted  posi- 
tion on  the  retina  ;  they  feel  and  handle  it,  and  find  it 
erect;  the  same  result  perpetually  recurs,  so  that,  at 
length,  long  before  they  can  reason  on  the  subject,  or 
even  describe  their  feelings  by  speech,  the  inverted 
images  give  them  an  idea  of  an  erect  object. 

C.  I  can  easily  conceive  that  this  would  be  the 
case  with  common  objects,  such  as  are  seen  every 
day  and  hour.  But  will  there  be  no  difficulty  in 
supposing  that  the  same  must  happen  with  regard 
to  any  thing  which  I  had  never  seen  before  ?  I  never 
saw  ships  sailing  on  the  sea  till  within  this  month  ; 
but  when  I  first  saw  them,  they  did  not  appear  to  me 
in  an  inverted  position. 

T.  But  you  have  seen  water  and  land  before,  and 
they  appear  to  you,  by  habit  and  experience,  to  be 
lowermost,  though  they  are  painted  on  the  eye  in  a 
different  position  :  and  the  bottom  of  the  ship  is  next 
the  water,  and  consequently,  as  you  refer  the  water  to 
the  bottom,  so  you  must  the  hull  of  the  ship  which  is 


OBJECTS  NOT  SEEN  DOUBLE.  391 


connected  with  it.  In  the  same  manner  all  the  parts 
of  a  distant  prospect  are  right  with  respect  to  each 
other  ;  and  therefore,  though  there  may  be  a  hundred 
objects  in  the  landscape  entirely  new  to  you,  yet  as 
they  all  bear  a  relation  to  one  another,  and  to  the 
earth  on  which  they  are,  you  refer  them,  by  experi- 
ence, to  an  erect  position. 

/.  How  is  it  that  in  so  small  a  space  as  the  retina 
of  the  eye,  the  images  of  so  many  objects  can  be 
formed? 

T.  Dr.  Paley*  tells  us,  The  prospect  from  Kamp- 
stead  Hill  is  compressed  into  the  compass  of  a  six- 
pence, yet  circumstantially  represented.  A  stage 
coach  travelling  at  its  ordinary  rate,  for  half  an  hour, 
passes  in  the  eye  only  over  one  twelfth  part  of  an 
inch,  yet  the  change  of  place  is  distinctly  perceived 
throughout  its  whole  progress."  Now  what  he  asserts 
we  all  know  is  true  :  go  to  the  window  and  look 
steadily  at  the  prospect  before  you,  and  see  how 
many  objects  you  can  discern  without  moving  your 
eye. 

/.  I  can  see  a  great  number  very  distinctly  indeed, 
besides  which  I  can  discern  others  on  both  sides, 
which  are  not  so  clearly  defined. 

C.  I  have  another  difficulty  ;  we  have  two  eyes,  on 
both  of  which  the  images  of  objects  are  painted';  how 
is  it  that  we  do  not  see  every  object  double  1 

T.  When  an  object  is  seen  distinctly  with  both 
eyes,  the  axes  of  them  are  directed  to  it,  and  the  object 
appears  single ;  for  the  optic  nerves  are  so  framed,  that 
the  correspondent  parts,  in  both  eyes,  lead  to  the  same 
place  in  the  brain,  and  excite  but  one  sensation.  But, 
if  the  axes  of  both  eyes  are  not  directed  to  the  object, 
that  object  seems  double. 

J,  How  does  that  appear  I 

*  See  Paley's  Natural  Theology,  p.  35,  7th  edit,  or 
p.  13,  in  the  Analysis  of  that  work,  by  the  Author  of 
these  Dialogues. 


3D2 


OPTICS. 


T.  Look  at  your  brother,  while  I  push  your  right 
eye  a  little  out  of  its  place  towards  the  left. 

J.  I  see  two  brothers,  the  one  receding  to  the  left 
hand  of  the  other. 

T.  The  reason  is  this ;  by  pushing  the  eye  out 
of  its  natural  place,  the  pictures  in  the  two  eyes  do 
not  fall  upon  correspondent  parts  of  the  retina,  and 
therefore  the  sensations  from  each  eye  are  excited  in 
different  parts  of  the  brain. 

When  any  point  of  an  object  is  seen  distinctly  with 
both  eyes,  the  axes  of  both  are  directed  to  that  point, 
and,  meeting  there,  the  object  appears  single,  though 
looked  at  with  both  eyes. 

Seeing  with  both  eyes  at  once  likewise  enables  us 
to  judge  more  accurately  of  distances  than  we  could 
if  we  saw  with  only  one. 

CONVERSATION  XVIT. 

OF   SPECTACLES,   AND   OF  THEIR  USES. 

C.  Why  do  people  wear  spectacles  ? 

T.  To  assist  the  sight,  which  may  be  defective  from 
various  causes.  Some  eyes  are  too  flat,  others  are 
too  convex  :  in  some  the  humours  lose  a  part  of  their 
transparency,  and  on  that  account  a  deal  of  light 
that  enters  the  eye  is  stopped  and  lost  in  the  passage, 
and  every  object  appears  dim.  The  eye,  without 
light,  would  be  a  useless  machine.  Spectacles  are 
intended  to  collect  the  light,  or  to  bring  it  to  a  proper 
degree  of  convergency. 

C.  Are  spectacle-glasses  always  convex  ? 

T.  No :  they  are  convex  when  the  eyes  are  too 
flat ;  but  if  the  eyes  are  already  very  convex,  then 
concave  glasses  are  used.  You  know  the  properties 
of  a  convex  glass  ? 

J.  Yes  ;  it  is  to  make  the  rays  of  light  converge 
sooner  than  they  would  without. 


OF  SPECTACLES. 


393 


f  


Fig.  28. 

T.  Suppose  then  a  person  is  unable  to  see  objects 
distinctly,  owing  to  the  cornea  c  d,  or  to  the  crystal- 
line a  h,  or  both,  being  too  flat.  The  focus  of  rays 
proceeding  from  any  object,  Xy  will  not  be  on  the 
retina,  where  it  ought  to  be,  but  at  z  beyond  it. 

C.  How  can  it  be  beyond  the  eye  1 

T,  It  would  be  beyond  it,  if  there  were  any  thing 
to  receive  it ;  as  it  is,  the  rays  flowing  from  x  will 
not  unite  at  so  as  to  render  vision  distinct.  To 
remedy  this,  a  glass  m  w  is  placed  between  the  object 
and  the  eye,  by  means  of  which  the  rays  are  brought 
to  a  focus  sooner,  and  the  image  is  formed  at  d. 

J.  Now  I  see  the  reason  why  people  are  obliged, 
sometimes,  to  make  trial  of  many  pairs  of  spectacles 
before  they  get  those  that  will  suit  them.  They  can- 
not tell  exactly  what  degree  of  convexity  is  necessary 
to  bring  the  focus  just  to  the  retina. 

T.  That  is  right ;  for  the  shape  of  the  eye  may  vary 
as  much  as  that  of  their  countenance ;  of  course,  a 
pair  of  spectacles  that  might  suit  you,  would  not  be 
adapted  to  another,  whose  eyes  should  require  a 
similar  aid. — What  is  the  property  of  concave 
glasses  1 

C.  They  cause  the  rays  of  light  to  diverge. 

T.  Then  for  very  round  and  globular  eyes,  these 
will  be  useful,  because,  if  the  cornea  c  d,  or  crystal- 
line humour  a  6,  be  too  convex,  the  rays  flowing  from 
X  will  unite  into  a  focus  before  they  arrive  at  the 
retina,  as  at  z, 

C.  If  the  sight  then  depend  on  sensations  produced 
on  the  retina,  such  a  person  will  not  see  the  object 
S2 


3:>1  OPTICS. 

at  all,  because  "the  image  of  it  does  not  reach  the 
retina. 


Fig.  29. 

T.  True :  t)ut  at  z  the  rays  cross  one  another, 
and  pass  on  to  the  retina,  where  they  will  produce 
some  sensations,  but  not  those  of  distinct  vision,  be- 
cause they  are  not  brought  to  a  focus  there.  To 
remedy  this,  the  concave  glass  m  n  is  interposed 
between  the  object  and  the  eye,  which  causes  the 
rays  coming  to  the  eye  to  diverge;  and  being  more 
divergent  when  they  enter  the  eye,  it  requires  a  very 
convex  cornea  or  crystalline  to  bring  them  to  a  focus 
at  the  retina. 

J.  I  have  seen  old  people,  when  examining  an 
object,  hold  it  a  good  distance  from  their  eyes. 

T.  Because  their  eyes  being  too  flat,  the  focus  is 
throvv'n  beyond  the  eye,  and  therefore  they  hold  the 
object  at  a  distance  to  bring  the  focus  z  (Fig.  28.)  to 
t'le  retina. 

C.  Very  short-sighted  people  bring  objects  close 
to  their  eyes. 

T,  Yes  ;  I  once  knew  a  young  man  who  was  apt, 
in  looking  at  his  paper,  to  rub  out  with  his  nose  what 
he  had  written  with  his  pen.  In  this  case,  bringing 
the  object  near  the  eye  produces  a  similar  effect  to 
that  produced  by  concave  glasses ;  because  the 
nearer  the  object  is  brought  to  the  eye,  the  greater 
is  the  angle  under  which  it  is  seen  ;  that  is,  the  ex- 
treme rays,  and,  of  course,  all  the  others,  are  made 
more  divergent. 

J .  I  do  not  understand  this. 


OF  SPECTACLES. 


895 


T.  Well,  then,  let  e  be  the  eye,  and  the  object 
a  b  seen  at  z,  and  also  at  .r,  double  the  distance  ;  will 


not  the  same  object  appear  under  different  angles  to 
an  eye  so  situated? 

J,  Yes,  certainly,  a  e  6  will  be  larger  than  c  e  J, 
and  will  include  it. 

T.  Then  the  object  being  brought  very  near  the 
eye,  has  the  same  effect  as  magnifying  the  object,  or 
of  causing  the  rays  to  diverge  ;  that  is,  though  a  b 
and  c  d  are  of  the  same  lengths,  yet  a  b  being  nearest 
to  the  eye  will  appear  the  largest. 

C.  YoQ  say  the  eyes  of  old  people  become  flat  by 
age  ;  is  that  the  natural  progress  ? 

T.  It  is  ;  and  therefore  people  who  are  very  short- 
sighted while  young,  will  probably  see  well  when 
they  grow  old. 

J.  That  is  an  advantage  denied  to  common  eyes. 

T.  But  people  blessed  with  common  sight,  should 
be  thankful  for  the  benefit  they  derived  while  young. 

C.  And  I  am  sure  we  cannot  too  highly  estimate 
the  science  of  optics,  that  has  afforded  such  assistance 
to  defective  eyes,  which,  in  many  circumstances  of 
life,  would  be  useless  without  them. 

T,  Salvinus  Armatus,  a  nobleman  of  Florence, 
claimed  the  honour  of  inventing  spectacles  ;  he  died 
iti  1317,  and  the  fact  was  inscribed  on  his  tomb.  But 
it  is  generally  allowed  that  Alhazen  was  really  the 
inventor,  about  50  years  before. 


Fig.  30. 


OPTICS. 


CONVERSATION  XVIII. 

OF  THE  RAINBOW. 

T.  You  have  frequently  seen  a  rainbow  ? 

C.  Oh,  yes;  and  very  often  there  are  two  at  the 
same  time,  one  above  the  other ;  the  lower  one  being 
by  far  the  most  brilliant. 

T.  This  is,  perhaps,  the  most  beautiful  meteor  in 
nature  ;  it  never  makes  its  appearance  but  when  a 
spectator  is  situated  between  the  sun  and  the  shower. 

/.  Is  a  rainbow  occasioned  by  the  falling  drops 
of  rain  1 

T.  Yes  ;  it  depends  on  the  reflection  and  refrac- 
tion of  the  rays  of  the  sun  by  the  falling  drops. 

C.  I  know  now  how  the  rays  of  the  sun  are  re- 
fracted by  water,  but  are  they  reflected  by  it  also  ? 

T,  Yes ;  water,  like  glass,  reflects  some  rays, 
while  it  transmits  or  refracts  others.  You  know  the 
beauty  of  the  rainbow  consists  in  its  colours. 

J.  Yes,  the  colours  of  the  rainbow"  is  a  very 
common  expression  ;  I  have  been  told  there  are  seven 
of  them,  but  it  is  seldom  that  so  many  can  be  clearly 
distinguished. 

r.  Perhaps  that  is  owing  to  your  want  of  patience  ; 
I  will  shew  you  the  colours  first  by  means  of  the  prism. 


Fig.  31. 

If  a  ray  of  light  s  be  admitted  into  a  darkened  room, 
through  a  small  hole  in  the  shutter  a  y,  its  natural 
coui-se  is  along  the  line  to  d :  but  if  a  glass  prism  a  c 
be  introduced,  the  whole  ray  will  be  bent  upwards, 


OF  THE  RAINBOW.  397 
and  if  it  be  taken  on  any  white  surface,  as  m  n,  it  will 
form  an  oblong  image  p  t,  the  breadth  of  which  is 
equal  to  the  diameter  of  the  hole  in  the  shutter. 

C.  This  oblong  is  of  different  colours  in  different 
parts. 

T.  These  are  the  colours  of  the  rainbow. 

/.  But  how  is  the  light  which  is  admitted  by  a 
circular  hole  in  the  window  spread  out  into  an 
oblong? 

T.  If  the  ray  were  of  one  substance,  it  would  be 
equally  bent  upwards,  and  make  only  a  small  circular 
image.  Since,  therefore,  the  image  or  picture  is 
oblong,  it  is  inferred,  that  it  is  formed  of  rays  differ- 
ently refrangible,  some  of  which  are  turned  more  out 
of  the  way,  or  more  upwards,  than  others  ;  those 
which  go  to  the  upper  part  of  the  spectrum  being 
most  refrangible,  those  which  go  to  the  lowest  part  are 
the  least  refrangible ;  the  intermediate  ones  possess 
more  or  less  refrangibility,  according  as  they  are 
painted  on  the  spectrum.  Do  you  see  the  seven 
colours  1 

C.  Yes ;  here  is  the  violet,  indigo,  blue,  green, 
yellow,  orange,  B.nd  red. 

T.  These  colours  will  be  still  more  beautiful  if  a 
convex  lens  be  interposed,  at  a  proper  distance,  be- 
tween the  shutter  and  the  prism. 

J.  How  does  this  apply  to  the  rainbow  \ 


Fig.  32. 


398 


OPTICS. 


T.  Suppose  A  to  be  a  drop  of  rain,  and  s  a  ray 
from  the  sun  falling  upon  or  entering  it  at  d,  it  will 
not  go  to  c,  but  be  refracted  to  n,  where  a  part  will 
go  out,  but  a  part  also  will  be  reflected  to  q,  where  it 
will  go  out  of  the  drop,  which,  acting  like  a  prism, 
separates  the  ray  into  its  primitive  colours,  and  the 
v  iolet  will  be  uppermost,  the  red  lowermost. 

C  Is  it  at  any  particular  angle  that  these  colours 
are  formed  1 

T.  Yes,  they  are  all  at  fixed  angles  ;  the  least  re- 
frangible, or  red,  makes  an  angle  with  the  solar  inci- 
dent ray,  equal  to  little  more  than  42  degrees  ;  and 
the  violet  or  most  refrangible  ray,  will  make  with  the 
solar  ray  an  angle  of  40  degrees. 

/.  I  do  not  understand  which  are  these  angles. 

T.  The  ray  s  d  would  go  to/ c,  therefore  the  angle 
made  with  the  red  ray  is  sfq,  and  that  made  with 
the  violet  ray  is  sc  q  ;  the  former  is  42°  2',  the  latter 
40«  17'. 

C.  Is  this  always  the  case  be  the  sun  either  high  or 
low  in  the  heavens  1 

T.  It  is  )  but  the  situation  of  the  rainbow  will  vary 
accordingly  as  the  sun  is  high  or  low,  that  is,  the 
higher  the  sun,  the  lower  will  be  the  rainbow  ;  a 
shower  has  been  seen  on  a  mountain  by  a  spectator 
in  a  valley,  by  which  a  complete  circular  rainbow 
has  been  exhibited. 

/.  And  I  once  remember  standing  on  Morant's  Court 
Ililj,  in  Kent,  when  there  was  a  heavy  shower,  while 
the  sun  shone  very  bright,  and  all  the  landscape  be- 
neath, to  a  vast  extent,  seemed  to  be  painted  with  the 
prismatic  colours. 

T.  I  recollect  this  well ;  it  was  certainly  the  most 
beautiful  one  I  ever  beheld. 

C.  You  have  not  explained  the  principles  of  the 
upper  or  fainter  bow. 

T,  This  is  formed  by  two  refractions  and  two 
reflections  :  suppose  the  ray  t  r  to  be  entering  the 
drop  B  at  r.  It  is  refracted  at  reflected  at  s,  re- 
flected again  at  i,  and  refracted  as  it  goes  out  at  u, 


OF  THE  RAINBOW. 


399 


v^lience  it  proceeds  being  separated  to  the  spectator  at 
g.  Here  the  colours  are  reversed  ;  the  angle  formed 
by  the  red  ray  is  51^  and  that  formed  by  the  violet 
is  540. 

J.  Does  the  same  thing  happen  with  regard  to  a 
whole  shower,  as  you  have  shewn  with  respect  to  the 
two  drops  ? 

T.  Certainly,  and  by  the  constant  falling  of  the 
rain  the  image  is  preserved  constant  and  perfect. 
Here  is  the  representation  of  the  two  bows.  The 


Fig.  33. 

rays  come  in  the  direction  s  a,  and  the  spectator 
stands  at  e  with  his  back  to  the  sun,  or,  in  other  words, 
he  must  be  between  the  sun  and  the  shower. 

This  subject  may  be  shewn  in  another  way  ;  if  a 
glass  globule  filled  with  water  be  hung  sufficiently 
high  before  you,  when  the  sun  is  behind,  to  appear 
red,  let  it  descend  gradually,  and  you  will  see,  in  the 
descent,  all  the  other  six  colours  follow  one  another. 
Artificial  rainbows  may  be  made  with  a  common 
watering  pot,  but  much  better  with  a  syringe  fixed  to 
an  artificial  fountain  ;  and  I  have  seen  one  by  spirt- 
ing up  water  from  the  mouth  :  it  is  often  seen  in 
cascades,  in  the  foaming  of  the  waves  of  the  sea,  in 
fountains,  and  even  in  the  dew  on  the  grass. 

Dr.  Langwith  has  described  a  rainbow,  v/hich  he 
saw  lying  on  the  ground,  the  colours  of  which  were 
almost  as  lively  as  those  of  the  common  rainbow.  It 
was  extended  several  hundred  yards,  and  the  colours 
were  so  strong,  that  it  might  have  been  seen  much 
farther,  if  it  had  not  been  terminated  by  a  bank,  and 
the  hedge  of  a  field. 

Rainbows  have  also  been  produced  by  the  reflec- 
tion of  the  sun's  beams  from  a  river  :   and  Mr, 


400 


OPTICS. 


Edwards  describes  one  which  must  have  been  formed 
by  the  exhalations  from  the  city  of  London,  when  the 
sun  had  been  set  twenty  minutes.* 


CONVERSATION  XIX. 


OF  THE   REFRACTING  TELESCOPE. 

T,  We  now  come  to  describe  the  structure  of 
telescopes,  of  which  there  are  two  kinds ;  viz.  the 
refracting  and  the  reflecting  telescope. 

C.  The  former  or  refracting  telescope  depends,  I 
suppose,  upon  leni^es  for  the  operation  ;  and  the  re- 
flecting telescope  acts  chiefly  by  means  of  mirrors. 

T.  Yes,  these  are  the  general  principles  upon 
which  they  are  formed  ;  and  we  shall  devote  this 
morning  to  the  explanation  of  the  refracting  telescope. 
Here  is  one  completely  fitted  up. 

J.  It  consists  of  two  tubes,  and 
two  glasses. 

T.  The  tubes  are  intended  to  hold 
the  glasses,  and  to  confine  the 
boundary  of  the  view.  I  will  there- 
fore explain  the  principle  by  the 
following  figure,  in  which  is  repre- 
sented the  eye  b,  the  two  lenses, 
mn,  oq,  and  the  object  xy.  The 
lens  0  q,  which  is  nearest  to  the 
object,  is  called  the  object-glass, 
and  that  m  n  nearest  to  the  eye  is 
called  the  eye-glass. 

C.  Is  the  object-glass  a  double 
convex,  and  the  eye-glass  a  double 
concave  ? 

T.  It  happens  so  in  this  particu- 
lar instance,  but  it  is  not  necessary 
that  the  eye-glass  should  be  concave  ; 
the  object-glass  must,  however,  in 
all  cases,  be  convex. 


Fig.  34. 


*  See  Phil.  Trans.  Vol.  VI.  and  L. 


REFRACTING  TELESCOPE.  401 

C.  I  see  exactly,  from  the  figure,  why  the  eye-glass 
is  concave :  for  the  convex  lens  converges  the  rays  too 
quickly,  and  the  focus  by  that  glass  alone  would  be  at 
E :  and  therefore  the  concave  is  put  near  the  eye  to 
make  the  rays  diverge  so  much  as  to  throw  them  to 
the  retina  before  they  come  to  a  focus. 

T.  But  that  is  not  the  only  reason:  by  coming  to 
a  focus  at  e,  the  image  is  very  small,  in  comparison  of 
what  it  is  when  the  image  is  formed  on  the  retina  by 
means  of  the  concave  lens.  Can  you,  James,  explain 
the  reason  of  all  the  lines  which  you  see  in  the  figure  ? 

J.  I  think  I  can ; — there  are  two  pencils  of  rays 
flowing  from  the  extremities  of  the  arrow,  which  is  the 
object  to  be  viewed.  The  rays  of  the  pencil  flowing 
from  X  go  on  diverging  till  they  reach  the  convex 
lens  0  q,  when  they  will  be  so  refracted  by  passing 
through  the  glass,  as  to  converge,  and  meet  in  the 
point  X,  Now  the  same  may  be  said  of  the  pencil  of 
rays  which  come  from  ^/j  and,  of  course,  of  all  the 
pencils  of  rays  flowing  from  the  object  between  x  and 
y.  So  that  the  image  of  the  arrow  would,  by  the  con- 
vex lens,  be  formed  at  e. 

T.  And  what  would  happen  if  there  were  no  other 
glass  ? 

J.  The  rays  would  cross  each  other  and  be  diver- 
gent, so  that  when  they  got  to  the  retina,  there  would 
be  no  distinct  image  formed,  but  every  point,  as  x  or  3/, 
would  be  spread  over  a  large  space,  and  the  image 
would  be  confused.  To  prevent  this  the  concave  lens 
m  n  is  interposed;  the  pencil  of  rays  which  would,  by 
the  convex  glass,  converge  at  x,  will  now  be  made  to 
diverge,  so  as  not  to  come  to  a  focus  till  they  arrive  at 
a ;  and  the  pencil  of  rays  which  would,  by  the  convex 
glass,  have  come  to  a  point  at  3/,  will,  by  the  interposi- 
tion of  the  concave  lens,  be  made  to  diverge  so  much 
as  to  throw  the  focus  of  the  rays  to  b  instead  oiy.  By 
this  means,  the  image  of  the  object  is  magnified. 

T.  Can  you  tell  the  reason  why  the  tubes  require  to 
be  drawn  out  more  or  less  for  different  persons  ? 

C.  The  tubes  are  to  be  adjusted  in  order  to  throw 


402 


OPTICS. 


the  focus  of  rays  exactly  on  the  retina:  and  as  some 
eyes  are  more  convex  than  others,  the  length  of  the 
focus  will  vary  in  different  persons,  and  by  sliding 
the  tube  up  and  down  this  object  is  obtained. 

T,  Refracting  telescopes  are  used  chiefly  for  viewing 
terrestrial  objects;  two  things,  therefore,  are  requisite 
in  them;  the  first  is,  that  it  should  shew  objects  in  an 
upright  position,  that  is,  in  the  same  position  as  we  see 
them  without  glasses ;  and  the  second  is,  that  they 
should  afford  a  large  field  of  view. 

J.  What  do  you  mean,  sir,  by  a  field  of  view? 

T.  All  that  part  of  a  landscape  which  may  be  seen  at 
once,  without  moving  the  eye  or  instrument.  Now  in 
looking  on  the  figure  again,  you  will  perceive  that  the 
concave  lens  throws  a  number  of  the  rays  beyond  the 
pupil  c  of  the  eye,  on  to  the  iris  on  both  sides,  but 
those  only  are  visible,  or  go  to  form  an  image,  which 
pass  through  the  pupil ;  and  therefore,  by  a  telescope 
made  in  this  way,  the  middle  part  of  the  object  is  only 
seen,  or,  in  other  words,  the  prospect  is  by  it  very 
much  diminished. 


J.  Is  not  the  image  of  the  object  in  [/ 
the  telescope  inverted  1  "^"^  " 

T.  Yes,  it  is :  for  you  see  the  image      Fig.  35. 


r.  By  substituting  a  double  con- 
vex eye-glass  g  h  instead  of  the  con- 
cave one.  Here  the  focus  of  the 
double  convex  lens  is  at  e,  and  the 
gkss  g  h  must  be  so  much  more  con- 
vex than  0  p,  as  that  its  focus  may  be 
also  at  E :  for  then  the  rays  flowing  from 
the  object  x  y,  and  passing  through  the 
object-glass  o  p,  will  form  the  inverted 
image  m  e  d.  Now  by  interposing  the 
double  convex  g  /i,the  image  is  thrown 
on  the  retina,  and  it  is  seen  under  the 
large  angle  dec,  that  is,  the  image 
7n  B  d  will  be  magnified  to  the  size 

C   E  D. 


C.  How  is  that  remedied  ? 


REFRACTING  TELESCOPE.  403 
on  the  retina  stands  in  tlie  same  position  as  the 
object ;  but  we  always  see  things  by  having  the 
images  inverted  :  and,  therefore,  whatever  is  seen 
by  telescopes  constructed  as  this  is,  will  appear  in- 
verted to  the  spectator,  which  is  a  very  unpleasant 
circumstance  with  regard  to  terrestrial  objects ;  it 
is  on  that  account  chiefly  used  for  celestial  obser- 
vations. 

C,  Is  there  any  rule  for  calculating  the  magnifying 
power  of  this  telescope  ? 

T.  It  magnifies  in  proportion  as  the  focal  distance 
of  the  object-glass  is  greater  than  the  focal  distance  of 
the  eye-glass.  Thus,  if  the  focal  distance  of  the  ob- 
ject-glass is  ten  inches,  and  that  of  the  eye-glass  only 
a  single  inch,  the  telescope  magnifies  the  diameter  of 
an  object  ten  times  ;  and  the  whole  surface  of  the  ob- 
ject will  be  magnified  a  hundred  times*. 

C.  Will  a  small  object,  as  a  silver  penny  for  instance, 
appear  a  hundred  times  larger  through  this  telescope 
than  it  would  by  the  naked  eye  1 

T.  Telescopes,  in  general,  represent  terrestrial  ob- 
jects to  be  nearer  and  not  larger:  thus,  looking  at  the 
silver  penny  a  hundred  yards  distant,  it  will  not 
appear  to  be  larger,  but  at  the  distance  only  of  a 
single  yard. 

J.  Is  there  no  advantage  gained  if  the  focal  dis- 
tance of  the  eye-glass  and  of  the  object-glass  be 
equal  ] 

T.  None  ;  and  therefore  in  telescopes  of  this  kind 
we  have  only  to  increase  the  focal  distance  of  the  ob- 
ject-glass, and  to  diminish  the  focal  distance  of  the 
eye-glass,  to  augment  the  magnifying  power  to  almost 
any  degree. 

C,  Can  you  carry  this  principle  to  any  extent  ? 

T.  Not  altogether  so :  an  object-glass  of  ten  feet 
focal  distance  will  require  an  eye-glass  whose  focal 
distance  is  rather  more  than  two  inches  and  a  half :  and 
an  object-glass  with  a  focal  distance  of  a  hundred 
feet,  must  have  an  eye-glass  whose  focus  must  be  about 


404 


OPTICS. 


six  inches  from  it.  How  much  will  each  of  these 
glasses  magnify? 

C.  Ten  feet  divided  by  two  inches  and  a  half,  give 
for  a  quotient  forty-eight;  and  a  hundred  feet  divided 
by  six  inches,  give  two  hundred :  so  that  the  former 
magnifies  48  times,  and  the  latter  200  times. 

T.  Refracting  telescopes  for  viewing  terrestrial  ob- 
jects, in  order  to  shew  them  in  their  natural  posture, 
are  usually  constructed  with  one  object-glass,  and 
three  eye-glasses,  the  focal  distances  of  these  last  being 
equal. 

J,  Do  you  make  use  of  the  same  method  in  calcu- 
lating the  magnifying  power  of  a  telescope  constructed 
in  this  way,  as  you  did  in  the  last  ? 

T.  Yes  ;  the  three  glasses  next  the  eye  having  their 
focal  distances  equal,  the  magnifying  power  is  found 
by  dividing  the  focal  distance  of  the  object-glass  by 
the  focal  distance  of  one  of  the  eye-glasses.  We  have 
now  said  as  much  on  the  subject  as  is  necessary  to  our 
plan. 

C.  What  is  the  construction  of  opera-glasses,  that 
are  so  much  used  at  the  theatre  ? 

T,  The  opera-glass  is  nothing  more  than  a  short  re- 
fracting telescope. 

The  lught  telescope  is  only  about  two  feet  long  :  it 
represents  objects  inverted,  much  enlightened,  but  not 
greatly  magnified.  It  is  used  to  discover  objects  not 
very  distant,  but  which  cannot  otherwise  be  seen  for 
want  of  sufficient  light. 

CONVEHSATION  XX. 

OF  REFLECTING  TELESCOPES. 

T,  This  is  a  telescope  of  a  different  kind,  and  is 
called  a  reflecting  telescope. 

C.  What  advantages  does  the  reflecting  telescope 
possess  over  that  which  you  described  yesterday? 

T,  The  great  inconvenience  attending  refracting 


REFLECTING  TELESCOPE.  405 
telescopes  is  their  length,  and  on  that  account  they 
are  not  very  much  used  when  high  powers  are  required. 
A  reflector  of  six  feet  long  will  magnify  as  much  as  a 
refractor  of  a  hundred  feet. 

J .  Are  these,  like  the  refractmg  telescopes,  made  in 
different  ways? 

T,  They  were  invented  by  Sir  I.  Newton,  but  have 
been  greatly  improved  since  his  time.  The  following 
figure  will  lead  to  a  description  of  one  of  those  most 


Fig.  36. 

in  use.  You  know  that  there  is  a  great  similarity 
between  convex  lenses  and  concave  mirrors. 

C.  They  both  form  an  inverted  focal  image  of  any 
remote  object,  by  the  convergence  of  the  pencils  of 
rays. 

T.  In  instruments,  the  exhibitions  of  which  are  the 
effects  of  reflection,  the  concave  mirror  is  substituted 
for  the  convex  lens,  t  t  represents  the  large  tube, 
and  t  t  the  small  tube  of  the  telescope,  at  one  end  of 
which  is  D  F,  a  concave  mirror,  with  a  hole  in  the 
middle  at  p,  the  principal  focus  of  which  is  at  i  k  ; 
opposite  to  the  hole  p  is  a  small  mirror  l,  concave 
towards  the  great  one  ;  it  is  fixed  on  a  strong  wire  m, 
and  may,  by  means  of  a  long  screw  on  the  outside  of 
the  tube,  be  made  to  move  backwards  or  forwards,  a  b 
is  a  reniote  object:  from  which  rays  will  flow  to  the 
great  mirror  d  f. 

/.  And  1  see  you  have  taken  only  two  rays  of  a 
pencil  from  the  top,  and  two  from  the  bottom. 

T.  And  in  order  to  trace  the  progress  of  the  reflec- 
tions and  refractions,  the  upper  ones  are  represented 
by  full  lines,  the  lower  ones  by  dotted  lines.   Now  the 


40G 


OPTICS. 


rays  at  c  and  e,  falling  upon  the  mirror  at  d  and  r, 
are  reflected,  and  form  an  inverted  image  at  m. 

C.  Is  there  anything  there  to  receive  the  image  ? 

T.  No  :  and  therefore  they  go  on  towards  the  re- 
flector L,  the  rays  from  different  parts  of  the  object 
crossing  one  another  a  little  before  they  reach  l. 

J.  Does  not  the  hole  at  p  tend  to  distort  the  image? 

T,  Not  at  all ;  the  only  defect  is,  that  there  is  less 
light.  From  the  mirror  l  the  rays  are  reflected  nearly 
parallel  through  p ;  there  they  have  to  pass  the  plano- 
convex lens  R,  which  causes  them  to  converge  at  a  h, 
and  the  image  is  now  painted  in  the  small  tube  near 
the  eye. 

C.  What  is  the  other  plano-convex  lens  s  for? 

T.  Having,  by  means  of  the  lens  i?,  and  the  two 
concave  mirrors,  brought  the  image  of  the  object  so 
nigh  as  at  a  h,  we  only  want  to  magnify  the  image. 

J.  This,  I  see,  is  done  by  the  lens  s. 

T.  It  is,  and  will  appear  as  large  as  c  d,  that  is,  the 
image  is  seen  under  the  angle  c  f  d. 

C.  How  do  you  estimate  the  magnifying  power  of 
the  reflecting  telescope? 

T,  The  rule  is  this:  '^Multiply  the  focal  distance 
of  the  large  mirror  by  the  distance  of  the  small  mirror 
from  the  image  m:  then  multiply  the  focal  distance  of 
the  small  mirror  by  the  focal  distance  of  the  eye-glass ; 
and  divide  these  two  products  by  one  another,  and  the 
quotient  is  the  magnifying  power." 

J.  It  is  not  likely  that  we  should  know  all  these  in 
any  instrument  we  possess. 

T.  The  following  then  is  a  method  of  finding  the 
same  thing  by  experiment.  **  Observe  at  what  dis- 
tance you  can  read  any  book  with  the  naked  eye, 
and  then  remove  the  book  to  the  farthest  distance  at 
which  you  can  distinctly  lead  by  means  of  the  tele- 
scope, and  divide  the  latter  by  the  former." 

C.  Had  not  Dr.  Herschel  a  very  large  reflecting 
telescope? 

r.  He  made  many,  but  the  tube  of  the  grand  tele- 
scope is  nearly  40  feet  long,  and  4  feet  ten  inches  in 


OF  THE  MICROSCOPE.  407 
diameter.  The  concave  surface  of  the  great  mirror  is 
48  inches  of  polished  surface  in  diameter,  and  it 
magnifies  6000  times.  This  noble  instrument  cost  the 
Doctor  four  years'  severe  labour ;  it  v^^as  finished 
August  28,  1789,  on  which  day  w^as  discovered  the 
sixth  satellite  of  Saturn. 


CONVERSATION  XXI. 

OF  THE  MICROSCOPE  ITS  PRINCIPLE  OF  THE  SINGLE 

MICROSCOPE  — OF  THE  COMPOUND    MICROSCOPE  OF 

THE   SOLAR  MICROSCOPE. 

T  We  are  nov^  to  describe  the  microscope,  which 
is  an  instrument  for  viewing  very  small  objects.  You 
know  that,  in  general,  persons  who  have  good  sight 
cannot  distinctly  view  an  object  at  a  nearer  distance 
than  about  six  inches. 

C.  I  cannot  read  a  book  at  a  shorter  distance  than 
this ;  but  if  I  look  through  a  small  hole  made  with  a 
pin  or  needle  in  a  sheet  of  brown  paper,  I  can  read  at 
a  very  small  distance  indeed. 

T.  You  mean,  that  the  letters  appear,  in  that  case, 
very  much  magnified,  the  reason  of  which  is,  that  you 
are  able  to  see  at  a  much  shorter  distance  in  this  way 
than  you  can  without  the  intervention  of  the  paper. 
Whatever  instrument,  or  contrivance,  can  render 
minute  objects  visible  and  distinct,  is  properly  a  mi- 
croscope. 

J.  If  I  look  through  the  hole  in  the  paper  at  the  dis- 
tance of  five  or  six  inches  from  the  print,  it  is  not 
magnified. 

r.  The  object  must  be  brought  near  to  increase  the 
angle  by  which  it  is  seen ;  this  is  the  principle  of  all 
microscopes,  from  the  single  lens  to  the  most  com- 
pound instrument,     a  is  an  object  not  clearly  visible 


Fi^/  37.  Fig.  38. 


408 


OPTICS. 


at  a  less  distance  than  a  b;  but  if  the  same  object  be 
placed  in  the  focus  c  of  the  lens  d,  the  rays  which 
proceed  from  it  will  become  parallel,  by  passing 
through  the  said  lens,  and  therefore  thcobject  is  dis- 
tinctly visible  to  the  eye  at  e,  placed  any  where  before 
the  lens.  There  are  three  distinctions  in  microscopes; 
the  single,  the  compound,  and  the  solar. 

C.  Does  the  single  microscope  consist  only  of  a 
lens? 

T.  By  means  of  a  lens  a  great  number  of  rays 
proceeding  from  a  point  are  united  in  the  same  sensi- 
ble point,  and  as  each  ray  carries  with  it  the  image  of 
the  point  from  whence  it  proceeded,  all  the  rays 
united  must  form  an  image  of  the  object. 

J,  Is  the  image  brighter  in  proportion  as  there  are 
more  rays  united? 

T.  Certainly:  and  it  is  more  distinct  in  proportion 
as  their  natural  order  is  preserved.  In  other  words, 
a  single  microscope  or  lens  removes  the  confusion  that 
accompanies  objects  when  seen  very  near  by  the  naked 
eye;  and  it  magnifies  the  diameter  of  the  object,  in 
proportion  as  the  focal  distance  is  less  than  the  limit 
of  distinct  vision,  which  we  may  reckon  from  about 
six  to  eight  inches. 

C.  If  the  focal  distance  of  a  reading-glass  be  four 
inches,  does  it  magnify  the  diameter  of  each  letter  only 
twice  1 

T.  Exactly  so  :  but  the  lenses  used  in  microscopes 
are  often  not  more  than  one-fourth,  or  one-eighth,  or 
even  one-twentieth  part  of  an  inch  radius. 

J.  And  in  a  double  convex  the  focal  distance  is 
always  equal  to  the  radius  of  convexity. 

T,  Then  tell  me  how  much  lenses  of  one-fourth, 
one-eighth,  and  one-twentieth  of  an  inch  will  each 
magnify  7 

/.  That  is  readily  done ;  by  dividing  8  inches,  the 
limit  of  distinct  vision,  by  one-fourth,  one-eighth,  and 
one-twentieth. 

C.  And  to  divide  a  whole  number,  as  8,  by  a  frac- 
tion, as  J,  &c.  is  to  multiply  the  said  number  by  the 


SINGLE  MICROSCOPE.  409 
denominator  of  the  fraction :  of  course,  8  multiplied 
by  4  gives  32 ;  that  is,  the  lens,  whose  radius  is  a 
quarter  of  an  inch,  magnifies  the  diameter  of  the 
object  32  times. 

/ .  Therefore  the  lenses  of  which  the  radii  are  one 
eighth,  and  one-twentieth,  v/ill  magnify  as  8  multi- 
plied by  8,  and  8  multiplied  by  20 ;  that  is,  the  for- 
mer will  magnify  64  times,  the  latter  160  times,  the 
diameter  of  an  object. 

T,  You  see,  then,  that  the  smaller  the  lens,  the 
greater  its  magnifying  power.    Dr.  Hooke  says,  in  his 
work  on  the  microscope,  that  he  has  made  lenses  so 
small  as  to  be  able,  not  only  to  distinguish  the  par- 
ticles of  bodies  a  million  times  smaller  than  a  visible 
pomt,  but  even  to  make  those  visible  of  which  a  mil- 
lion times  a  million  wojild  hardly  be  equal  to  the 
bulk  of  the  smallest  grain  of  sand. 
C.  I  wonder  how  he  made  them. 
T,  I  will  give  you  his  description :  he  first  took  a 
very  narrow  and  thin  slip  of  clear  glass,  melted  it  in 
the  flame  of  a  candle  or  lamp,  and  drew  it  out  into 
exceedingly  fine  threads.    The  end  of  one  of  these 
threads  he  melted  again  in  the  flame  till  it  run  into  a 
very  small  drop,  which,  when  cool,  he  fixed  in  a  thin 
plate  of  metal,  so  that  the  middle  o/  it  might  be 
directly  over  the  centre  of  an  extremely  small  hole 
made  in  the  plate.    Here  is  a  very  convenient  single 
microscope. 

/.  It  does  not  seem,  at  first  sight,  so  simple  as 
those  which  you  have  just  now  described. 


Fig.  39. 
T 


410  OPTICS. 

T.  A  is  a  circular  piece  of  brass,  or  it  may  be  made 
of  wood,  ivory,  &c.  in  the  middle  of  which  is  a  very 
small  hole  ;  in  this  is  fixed  a  small  lens,  the  focal 
distance  of  which  is  o  d  ;  at  that  distance  is  a  pair  of 
pliers  DE,  which  may  be  adjusted  by  the  sliding 
screw  p,  and  opened  by  means  of  two  little  studs  a  e ; 
with  these  any  small  object  may  be  taken  up,  and 
viewed  with  the  eye  placed  in  the  other  focus  of  the 
lens  at  f,  to  which  it  will  appear  magnified  as 
at  iM. 

C.  T  see  by  the  joint  it  is  made  to  fold  up. 

r.  It  is  ;  and  may  be  put  into  a  case,  and  carried 
about  in  the  pocket,  without  any  incumbrance  or  in- 
convenience. Let  us  now  look  at  a  double  or  com- 
pound microscope. 

J.  How  many  glasses  are  there  in  this  1 

T.  There  are  two  ;  and  the  construction  of  it  may 
be  seen  by  this  figure  ;  cd  is  called 
the  object-glass,  and  e/the  eye-glass 
The  small  object  ab  is  placed  a  little 
farther  from  the  glass  cd  than  its  prin- 
.  cipal  focus,  so  that  the  pencils  of  rays 
flowing  from  the  different  points  of  the 
object,  and  passing  through  the  glass, 
may  be  made  to  converge  and  unite 
in  as  many  points  between  g  and  h, 
where  the  image  of  the  object  will  be 
formed.  This  image  is  viewed  by  the 
eye-glass,  ef,  which  is  so  placed  that 
the  image  g  h  may  be  in  the  focus, 
and  the  eye  at  about  an  equal  dis- 
tance on  the  other  side  ;  the  rays  of  ^ 
each  pencil  will  be  parallel  after  going  out  of  the 
eye-glass,  as  at  e  and  f,  till  they  come  to  the  eye  at 
k,  by  the  humours  of  which  they  will  be  converged 
and  collected  into  points  on  the  retina,  and  form  the 
large  inverted  image  a  b. 

C.  Pray,  sir,  how  do  you  calculate  the  magnifying 
power  of  this  microscope  ? 

r.  There  are  two  proportions  which,  when  found, 


Fio'.  40. 


COMPOUND  MICROSCOPE.  411 

are  to  be  multiplied  into  one  another:  (1)  As  the 
distance  of  the  image  from  the  object-glass  is  greater 
than  its  distance  from  the  eye-glass  ;  and,  (2)  as 
the  distance  from  the  object  is  less  than  the  limit  of 
distinct  vision. 

Example.  If  the  distance  of  the  image  from  the 
object-glass  be  4  times  greater  than  from  the  eye- 
glass, the  magnifying  power  of  4  is  gained  ;  and  if 
the  focal  distance  of  the  eye-glass  be  one  inch,  and 
the  distance  of  distinct  vision  be  considered  at  7 
inches,  the  magnifying  power  of  7  is  gained,  and 
7x4  gives  28  ;  that  is,  the  diameter  of  the  object 
will  be  magnified  28  times,  and  the  surface  will  be 
magnified  784  times. 

J.  Do  you  mean  that  an  object  will,  through  such 
a  microscope,  appear  784  times  larger  than  by  the 
naked  eye  ? 

T.  Yes,  I  do  ;  provided  the  limit  of  distinct  vision 
be  7  inches ;  but  some  persons,  who  are  short- 
sighted, can  see  as  distinctly  at  5  or  4  inches  as 
another  can  at  7  or  8 ;  to  the  former  the  object  will 
not  appear  so  large  as  to  the  latter. 

Ex.  2.  What  will  a  microscope  of  this  kind  mag- 
nify to  three  different  persons,  whose  eyes  are  so 
formed  as  to  see  distinctly  at  the  distance  of  6,  7,  and 
8  inches  by  the  naked  eye ;  supposing  the  image  of 
the  object-glass  to  be  five  times  as  distant  as  from  the 
eye-glass,  and  the  focal  distance  of  the  eye-glass  be 
only  the  tenth  part  of  an  inch  1 

C.  As  five  is  gained  by  the  distances  between  the 
glasses,  and  60,  70,  and  80,  by  the  eye-glass,  the 
magnifying  powers  will  be  as  300,  350,  and  400. 

J.  How  is  it  that  60,  70,  and  80,  are  gained  by 
the  eye-glass  1 

C.  Because  the  distances  of  distinct  vision  are  put 
at  6,  7,  and  8  inches,  and  these  are  to  be  divided  by 
the  focal  distance  of  the  eye-glass,  or  by  one-tenth  ; 
but  to  divide  a  whole  number  by  a  fraction,  we  must 
multiply  that  number  by  the  denominator,  or  lower 
figure  in  the  fraction  :  therefore,  the  power  gained  by 


412 


OFrics. 


the  distance  between  the  two  glasses,  or  5,  must  be 
multiplied  by  60,  70,  or  80.  And  the  surface  of  the 
object  will  be  magnified  in  proportion  to  the  square 
of  300,  350,  or  400,  that  is,  as  90,000,  122,600,  or 
160,000. 

T.  We  come  now  to  the  solar  microscope,  which 
is  by  far  the  most  entertaining  of  them  all,  because 
the  image  is  much  larger,  and  being  thrown  on  a 
sheet,  or  other  white  surface,  may  be  viewed  by  many 
spectators  at  the  same  time,  without  any  fatigue  to 
the  eye.  Here  is  one  fixed  in  the  window  shutter ; 
but  I  can  explain  its  construction  best  by  a  figure. 

J.  There  is  a  looking-glass  on  the  outside  of  the 
window. 


Fig.  41. 


T,  Yes,  the  solar  microscope  consists  of  a  looking- 
glass  s  0  without,  the  lens  a  h  in  the  shutter  d  u,  and 
the  lens  nm  within  the  dark  room.  These  three 
parts  are  united  to,  and  in  a  brass  tube.  The  looking- 
glass  can  be  turned  by  the  adjusting-screw,  so  as  to 
receive  the  incident  rays  of  the  sun  s  s  s,  and  reflect 
them  through  the  tube  into  the  room.  The  lens  a  h 
collects  those  rays  into  a  focus  at  n  m,  where  there  is 
another  magnifier  ;  here,  of  course,  the  rays  cross, 
and  diverge  to  the  white  screen  on  which  the  image 
of  the  object  will  be  painted. 

C.  I  see  the  object  is  placed  a  little  behind  the 
focus. 

2'.  If  it  were  m  the  focus  it  would  be  burnt  to 


CAMERA  OBSCURA. 


413 


pieces  immediately.  The  magnifying  power  of  this 
instrument  depends  on  the  distance  of  the  sheet  or 
white  screen  ;  perhaps  about  10  feet  is  as  good  a 
distance  as  any.  You  perceive,  that  the  size  of  the 
image  is  to  that  of  the  object  as  the  distance  of  the 
former  from  the  lens  n  m  is  to  that  of  the  latter. 

J.  Then  the  nearer  the  object  to  the  lens,  and  the 
farther  the  screen  from  it,  the  greater  the  power  of 
this  microscope. 

T,  You  are  right,  and  if  the  object  be  only  half  an 
inch  from  the  lens,  and  the  screen  nine  feet,  the 
image  will  be  46,656  times  larger  than  the  object :  do 
you  understand  this  1 

C,  Yes ;  the  object  being  only  half  an  inch  from 
the  lens,  and  the  image  9  feet,  or  108  inches,  or  216 
half  inches,  the  diameter  of  the  image  will  be  216 
times  larger  than  the  diameter  of  the  object,  and  this 
number  multiplied  into  itself  will  give  46,656. 

r.  This  instrument  is  calculated  only  to  exhibit 
transparent  objects,  or  such  as  the  light  can  pass 
through  in  part.  For  opaque  objects  a  different  mi- 
croscope is  used  :  and  indeed  there  are  an  indefinite 
number  of  microscopes. 


CONVERSATION  XXII. 

OF  THE    CAMERA    OBSCURA,    MAGIC   LANTHORN,  AND 
MULTIPLYING  GLASS. 

T.  We  shall  now  treat  upon  some  miscellaneous 
subjects;  of  which  the  first  shall  be  the  Camera  Ob' 
scura. 

C.  What  is  a  camera  obscura  ? 

T.  The  meaning  of  the  term  is  a  darkened  cham- 
ber :  the  construction  of  it  is  very  simple,  and  will  be 
understood  in  a  moment  by  you,  who  know  the  pro- 
perties of  the  convex  lens. 

A  convex  lens  placed  in  a  hole  of  a  window-shutter 
will  exhibit,  on  a  white  sheet  of  paper  placed  in  tlie 


414 


OPTICS. 


focus  of  the  glass,  all  the  objects  on  the  outside,  as 
fields,  trees,  men,  houses,  &c.  in  an  inverted  order. 

J.  Is  the  room  to  be  quite  dark,  except  the  light 
which  is  admitted  through  the  lens  1 

T,  It  ought  to  be  so ;  and  to  have  a  very  interest- 
ing picture,  the  sun  should  shine  upon  the  objects. 

J.  Is  there  no  other  kind  of  camera  obscura  ? 

T.  A  portable  one  may  be  made  with  a  square 
box,  in  one  side  of  vhich  is  to  be  fixed  a  tube,  having 
a  convex  lens  in  it :  within  the  box  is  a  plane  mirror 
reclining  backwards  from  the  tube,  in  an  angle  of 
forty-five  degrees. 

C.  On  what  does  this  mirror  reflect  the  image  of 
the  object  1 

T,  The  top  of  the  box  is  a  square  of  unpolished 
glass,  on  which  the  picture  is  formed.  And  if  a  piece 
of  oiled  paper  be  stretched  on  the  glass,  a  landscape 
may  be  easily  copied  j  or  the  outline  may  be  sketched 
on  the  rough  surface  of  the  glass. 

/.  Why  is  the  mirror  to  be  placed  at  an  angle  of 
45  degrees  exactly  1 

T,  The  image  of  the  objects  would  naturally  be 
formed  at  the  back  of  the  box  opposite  to  the  lens ;  in 
order,  therefore,  to  throw  it  on  the  top,  the  mirror 
must  be  so  placed  that  the  angle  of  incidence  shall 
be  equal  to  the  angle  of  reflection.  In  the  box,  ac- 
cording to  its  original  make,  the  top  is  at  right  angles 
to  the  end,  that  is  at  an  angle  of  90  degrees,  therefore 
the  mirror  is  put  at  half  90,  or  45  degrees. 

C.  Now  the  incident  rays  falling  upon  a  surface 
which  declines  to  an  angle  of  45  degrees,  will  be  re- 
flected at  an  equal  angle  of  45  degrees,  which  is  the 
angle  that  the  glass  top  of  the  box  bears  with  respect 
to  the  mirror. 

J.  If  I  understand  you  clearly,  had  the  mirror  been 
placed  at  the  end  of  the  box  or  parallel  to  it,  the  rays 
would  have  been  reflected  back  to  the  lens  ;  and 
none  would  have  proceeded  to  the  top  of  the  box. 

T.  True  :  in  the  same  manner  as  when  one  person 
stands  before  a  looking-glass,  another  at  the  side  of 


MAGIC  LANTHORN. 


'415 


the  room  cannot  see  his  image  in  the  glass,  because 
the  rays  flowing  from  him  to  the  looking-glass  are 
thrown  back  to  himself  again  j  but  let  each  person 
stand  on  the  opposite  side  of  the  room,  while  the 
glass  is  in  the  middle  of  the  end  of  it,  they  will  both 
stand  at  an  angle  of  45  degrees,  with  regard  to  the 
glass,  and  the>jays  from  each  will  be  reflected  to  the 
other. 

C.  Is  the  tube  fixed  in  this  machine  1 
T,  No ;  it  is  made  to  draw  out,  or  push  in,  so  as  to 
adjust  the  distance  of  the  convex  glass  from  the  mir- 
ror, in  proportion  to  the  distance  of  the  outward  ob- 
jects, till  they  are  distinctly  painted  on  the  horizontal 
glass. 

J.  Will  you  now  explain  the  structure  of  the  magic 
lanthorn,  which  has  long  afforded  us  occasional 
amusement '? 

T,  This  little  machine  consists,  as  you  know,  of  a 
sort  of  tin  box ;  within  which  is  a  lamp  or  candle  : 
the  light  of  this  passes  through  a  great  plano-convex 
lens,  placed  in  a  tube  fixed  in  the  front.  This  strongly 
illuminates  the  objects,  which  are  painted  on  slips  of 
glass,  and  placed  before  the  lens  in  an  inverted  posi- 
tion. A  sheet,  or  other  white  surface,  is  placed  to  re- 
ceive the  images. 

C.  Do  you  invert  the  glasses  on  which  the  figures 
are  drawn,  in  order  that  the  images  of  them  may  be 
erect  1 

T.  Yes  :  and  the  illumination  may  be  greatly  in- 
creased, and  the  effect  much  more  powerful,  by 
placing  a  concave  mirror  at  the  back  of  the  lamp. 

C.  Did  you  not  tell  us  that  the  Phantasmagoria, 
which  we  saw  at  the  Lyceum,  was  a  species  of  the 
magic  lanthorn  1 

T.  There  is  this  difference  between  them  :  in  com- 
mon magic  lanthorns,  the  figures  are  painted  on  tran- 
sparent glass,  consequently  the  image  on  the  screen 
is  a  circle  of  light,  having  a  figure  or  figures  on  it : 
but  in  the  Phantasmagoria,  all  the  glass  is  made 
opaque,  except  the  figure  only,  which  being  painted  in 


41G  OPTICS, 
transparent  colours,  the  light  shines  through  it,  and 
no  light  can  come  upon  the  screen  but  what  passes 
through  the  figure. 

J.  But  there  was  no  sheet  to  receive  the  picture. 

T.  No  ;  the  representation  was  thrown  on  a  thin 
screen  of  silk  placed  between  the  spectators  and  the 
lanthorn. 

C.  What  caused  the  images  to  appear  approaching 
and  receding? 

T.  It  is  owing  to  removing  the  lanthorn  farther 
from  the  screen,  or  bringing  it  nearer  to  it ;  for  the 
size  of  the  image  must  increase,  as  the  lanthorn  is 
carried  back,  because  the  rays  come  in  the  shape  of 
a  cone ;  and  as  no  part  of  the  screen  is  visible,  the 
figure  appears  to  be  formed  in  the  air,  and  to  move 
farther  off  when  it  becomes  smaller,  and  to  come 
nearer  as  it  increases  in  size. 

J.  Here  is  another  instrument,  the  construction  of 
which  you  promised  to  explain:  the  multiplying 
glass* 

jT.  One  side  of  this  glass  is  cut  into  many  distinct 
surfaces,  and  in  looking  at  an  object,  as  your  brother, 
through  it,  you  will  see  not  one  object  only,  but  as 
many  as  the  glass  contains  plane  surfaces. 

I  will  draw  a  figure  to  illustrate  _  _ 
this  :  let  AB  represent  a  glass,  flat 
at  the  side  next  the  eye  h,  and  cut 
into  three  distinct  surfaces  on  the 
opposite  side,  as  a  h,  h  d,  d  b.  The 
object  c  will  not  appear  magnified, 
but  as  rays  will  flow  from  it  to  all 
parts  of  the  glass,  and  each  plane 
surface  will  refract  these  rays  to  the  eye,  the  same 
object  will  appear  to  the  eye  in  the  direction  of  the 
rays  which  enter  it  through  each  surface.  Thus  a  ray 
c  /  falling  perpendicularly  on  the  middle  surface,  will 
suffer  no  refraction,  but  shew  the  object  in  its  true 
place  at  c :  the  ray  from  c  b  falling  obliquely  on 
the  plane  surface  a  h,  will  be  refracted  in  the  direc- 
tion be,  and  on  leaving  the  glass  at  e,  it  will  pass  to 


J 


MULTIPLYING  GLASS  417 
the  eye  in  the  direction  of  e  h,  and  therefore  it  appears 
at  e:  and  the  ray  cd  will,  for  the  same  reason,  be 
refracted  to  the  eye  in  the  direction  b  h,  and  the  object 
c  will  appear  also  in  d. 

If  instead  of  three  sides,  the  glass  had  been  cut 
into  6  or  20,  there  would  have  appeared  6  or  20 
different  objects  differently  situated. 


T2 


MAGNETISM. 


CONVERSATION  I. 

OF  THE  MAGNET  J  ITS  PROPERTIES  ;  USEFUL  TO  MARI- 
NERS, AND  others;  iron  rendered  magnetic; 

PROPERTIES  OF  THE  MAGNET. 

TUTO  R  CH  A  R  LES  JAMES . 

Tutor.  You  see  this  dark  brown  mineral  body  ;  it 
is  almost  black,  and  you  know  it  has  the  property  of 
attracting  needles  and  other  small  iron  substances. 

James.  Yes,  it  is  called  a  load-stone,  leading-stone, 
or  magnet ;  we  have  often  been  amused  with  it ;  but 
you  told  us  that  it  possessed  a  much  more  important 
property  than  that  of  attracting  iron  and  steel. 

T.  This  is  what  is  called  the  directive  property,  by 
which  mariners  are  enabled  to  conduct  their  vessels 
through  the  mighty  ocean  out  of  the  sight  of  land  ; 
by  the  aid  of  this,  miners  are  guided  in  their  subter- 
ranean inquiries,  and  the  traveller  through  deserts 
otherwise  impassable. 

Charles.  Were  not  mariners  unable  to  make  long 
and  very  distant  voyages  till  this  property  of  the 
magnet  was  discovered'; 

T.  Till  then,  they  contented  themselves  with  mere 
coasting  voyages :  seldom  trusting  themselves  from  the 
sight  of  land. 

J.  How  long  is  it  since  this  property  of  the  magnet 
was  first  known  ? 

T.  About  five  hundred  years;  and  it  is  not  possible 
to  ascertain,  with  any  degree  of  precision,  to  whom 
we  are  indebted  for  this  great  discovery 

C.  You  have  not  told  us  in  what  the  discovery 
consists. 

T.  When  a  magnet,  or  a  needle  rubbed  with  a 
magnet,  is  freely  suspended,  it  will  always,  and  in  all 


USE  OF  THE  MAGNET.  419 
places,  stand  nearly  north  and  south  ;  and  its  devia- 
tion from  either  point,  at  any  one  place,  remains  the 
same,  within  narrow  limits,  for  a  long  time. 

C.  Is  it  known  which  end  points  to  the  north,  and 
which  to  the  south? 

T,  Yes,  or  it  would  be  of  little  use :  each  magnet, 
and  each  needle,  or  other  piece  of  iron,  that  is  made 
an  artificial  magnet  by  being  properly  rubbed  with  the 
natural  magnet,  has  a  north  end  and  a  south  end, 
called  the  north  and  south  'poles :  to  the  former  a  mark 
is  placed,  for  the  purpose  of  distinguishing  it. 

/ .  Then  if  a  ship  were  to  make  a  voyage  to  the 
north,  it  must  follow  the  direction  which  the  magnet 
takes  ;  making  a  due  allowance  for  the  deviation  you 
mentioned. 

T.  True ;  and  if  it  were  bound  a  westerly  course, 
the  needle  always  pointing  north,  the  ship  must  keep 
in  a  direction  at  right  ang-les  to  the  needle.  In  other 
words,  the  direction  of  the  needle  must  be  across 
the  ship. 

C.  Could  not  the  same  object  be  obtained  by  means 
of  the  pole  star? 

r.  It  might,  in  a  considerable  degree,  provided  you 
could  always  insure  a  fine  clear  sky  ;  but  what  is  to 
be  done  in  cloudy  weather,  which,  in  some  latitudes, 
will  last  for  many  days  together? 

C.  I  did  not  think  of  that. 

T.  Without  the  use  of  the  magnet,  no  persons 
could  have  ventured  upon  such  voyages  as  those  to 
the  West  Indies,  and  other  distant  parts;  the  know- 
ledge, therefore,  of  this  instrument  cannot  be  too 
highly  prized. 

/.  Is  that  a  magnet  which  is  fixed  to  the  bottom  of 
the  globe,  by  means  of  which  we  set  the  globe  in  a 
proper  direction  with  regard  to  the  cardinal  points, 
north,  south,  east,  and  west  ? 

T.  That  is  called  a  compass,  the  needle  of  which, 
being  rubbed  by  the  natural  or  real  magnet,  becomes 
possessed  of  the  same  properties  as  those  which  belong 
to  the  magnet  itself. 


420 


MAGNETISM. 


C.  Can  any  iron  and  steel  be  made  magnetic? 

T.  They  may  ;  but  iron  is  the  most  proper  for  the 
purpose.  Bars  of  iron  thus  prepared  are  called  a7^ti- 
ficial  magnets. 

J.  Will  these  soon  lose  the  properties  thus  ob- 
tained 1 

T.  Artificial  magnets  will  retain  their  properties 
almost  any  length  of  time,  and  since  they  may  be 
rendered  more  powerful  than  natural  ones,  and  can 
be  made  of  any  form,  they  are  generally  used,  so  that 
the  natural  magnet  is  kept  more  as  a  curiosity  than 
for  utility. 

C.  What  are  the  leading  properties  of  the  magnet  1 
T.  (1.)  A  magnet  attracts  iron.  (2.)  When  placed 
so  as  to  be  at  liberty  to  move  in  any  direction,  its 
north  end  points  to  the  north  pole,  and  its  south  end 
to  the  south  pole :  this  is  called  the  polarity  of  the 
magnet.  (3.)  When  the  north  pole  of  one  magnet 
is  presented  to  the  south  pole  of  another,  they  will 
attract  one  another.  But  if  the  two  soiith  or  the  two 
7iorth  poles  are  presented  to  each  other,  they  will  repel. 
(4.)  When  a  magnet  is  so  situated  as  to  be  at  liberty 
to  move  any  way,  the  two  poles  of  it  do  not  lie  in  an 
horizontal  direction ;  it  inclines  one  of  its  poles  towards 
the  horizon,  and,  of  course,  elevates  the  other  pole 
above  it ;  this  is  called  the  inclination  or  dipping  of 
the  magnet.  (5.)  Any  magnet  may  be  made  to 
impart  its  properties  to  iron  and  steel. 

But  the  properties  of  the  magnet  will  be  enlarged 
upon  during  our  future  Conversations. 

CONVERSATION  II. 

MAGNETIC  ATTRACTION  AND  REPULSION. 

T.  Having  mentioned  the  several  properties  of  the 
magnet  or  loadstone,  I  intend,  at  this  time,  to  enter 
more  particularly  into  the  nature  of  magnetic  attrac- 
tion and  repulsion. — Here  is  a  thin  iron  bar,  eight  or 
nine  inches  long,  rendered  magnetic,  and  on  that  ac- 


ATTRACTION,  &c. 


421 


count  it  is  now  called  an  artificial  magnet :  I  bring 
a  small  piece  of  iron  within  a  little  distance  of  one  of 
the  poles  of  the  magnet,  and  you  see  it  is  attracted  or 
drawn  to  it. 

C.  Will  not  the  same  effect  be  produced,  if  the 
iron  be  presented  to  any  other  part  of  the  magnet  ? 

T.  The  attraction  is  strongest  at  the  poles,  and  it 
grows  less  and  less  in  proportion  to  the  distance  of 
any  part  from  the  poles ;  so  that  in  the  middle,  between 
the  poles,  there  is  no  attraction,  as  you  shall  see  by 
means  of  this  large  needle. 

J.  When  you  held  the  needle  near  the  pole  of  the 
magnet,  the  magnet  moved  to  that,  which  looks  as  if 
the  needle  attracted  the  magnet. 

T,  You  are  right:  the  attraction  is  mutual,  as  is 
evident  from  the  following  experiment.  T  place  this 
small  magnet  on  a  piece  of  cork,  and  the  needle  on 
another  piece,  and  let  them  float  on  water,  at  a  little 
distance  from  each  other,  and  you  observe  that  the 
magnet  moves  towards  the  iron,  as  much  as  the  iron 
moves  towards  the  magnet. 

C.  If  two  magnets  were  put  in  this  situation,  what 
would  be  the  effect? 

r.  If  poles  of  the  same  name,  that  is,  the  two 
north,  or  the  two  south,  be  brought  near  together, 
they  will  repel  one  another ;  but  if  a  north  and  south 
be  presented,  the  same  kind  of  attraction  will  be 
visible,  as  there  was  between  the  magnet  and  needle. 

J.  Will  there  be  any  attraction  or  repulsion  if 
other  bodies,  as  paper  or  thin  slips  of  wood,  be 
placed  between  the  magnets,  or  between  the  magnet 
and  iron? 

r.  Neither  the  magnetic  attraction  nor  repulsion 
is  in  the  least  diminished,  or  in  any  way  affected,  by 
the  interposition  of  any  kind  of  bodies,  except  iron. 
Bring  the  magnets  together  within  the  attracting  or 
repelling  distance,  and  hold  a  slip  of  wood  between 
them  ;  you  see  they  both  come  to  the  wood. 

C,  You  said  that  iron  was  more  easily  rendered 


422 


MAGNETISM. 


magnetic  than  steel;  does  it  retain  the  properties  as 
long  too  1 

T.  If  a  piece  of  soft  iron  and  a  piece  of  hard 
steel  be  brought  within  the  influence  of  a  magnet, 
the  iron  will  be  most  forcibly  attracted,  but  it  will 
almost  instantly  lose  its  acquired  magnetism,  whereas 
the  hard  steel  will  preserve  it  a  long  time. 

J,  Is  magnetic  attraction  and  repulsion  at  all  like 
what  we  have  sometimes  seen  in  electricity? 

T.  In  some  instances  there  is  a  great  similarity : 
Ex.  I.    Tie  two  pieces  of  soft  wire  each 
to  a  separate  thread,  which  join  at  top,  ^ 
and  let  them  hang  freely  from  a  hook  x,       11"'  ' 
If  I  bring  the  marked  or  north  end  of  a  I 
magnetic  bar  just  under  them,  you  will      i  \ 
see  the  wires  repel  one  another,  as  they     j  \ 
are  shewn  in  the  figure  hanging  from  s.        |  ||  -j, 

C.  Is  that  occasioned  by  the  repelling     |j  i 
power  which  both  wires  have  acquired  in     1"    1  I 
consequence  of  being  both  rendered  mag- 
netic with  the  same  pole  1  Fig.  1 . 

T,  It  is  :  and  the  same  thing  would 
have  occurred  if  the  south  pole  had  been  presented 
instead  of  the  north. 

/.  Will  they  remain  long  in  that  position  ? 

T.  If  the  wires  are  of  very  soft  iron  they  will 
quickly  lose  their  magnetic  power  ;  but  if  steel  wires 
be  used,  as  common  sewing  needles,  they  will  con- 
tinue to  repel  each  other  after  the  removal  of  the 
magnet. 

Ex.  II.  I  lay  a  sheet  of  paper  flat  upon  a  table, 
and  strew  some  iron  filings  upon  it.  I  now  lay  this 
small  magnet  among  them,  and  give  the  table  a  few 


IRON  MAGNETIC. 


423 


gentle  knocks,  so  as  to  shake  the  filings,  and  you  ob- 
serve in  what  manner  they  have  arranged  themselves 
about  the  magnet. 

C.  At  the  two  ends  or  poles  the  particles  of  iron 
form  themselves  into  lines,  a  little  sideways  ;  they 
bend,  and  then  form  complete  arches,  reaching  from 
some  point  in  the  northern  half  of  the  magnet  to 
some  other  point  in  the  southern  half. — Pray  how  do 
you  account  for  this  1 

T,  Each  of  the  particles  of  iron,  by  being  brought 
within  the  sphere  of  the  magnetic  influence,  becomes 
itself  magnetic,  and  possessed  of  two  poles,  and  con- 
sequently disposes  itself  in  the  same  manner  as  any 
other  magnet  would  do,  and  also  attracts  with  its 
extremities  the  contrary  poles  of  other  particles. 

Ex.  III.  If  I  shake  some  iron  filings  through  a 
gauze  sieve,  upon  a  paper  that  covers  a  bar  magnet, 
the  filings  will  become  magnets,  and  will  be  arranged 
in  beautiful  curves. 

J.  Does  the  polarity  of  the  magnet  reside  only  in 
the  two  ends  of  its  surface  ? 

T.  No  :  one  half  of  the  magnet  is  possessed  of 
one  kind  of  polarity,  and  the  other  of  the  other  kind, 
but  the  ends,  or  poles,  are  those  points  in  which  that 
power  is  the  strongest. 

Definition.  A  line  drawn  from  one  pole  to  the 
other  is  called  the  axis  of  the  magnet. 


CONVERSATION  III. 

the  method  of  making  magnets  of  the 

mariner's  compass. 

r.  I  have  already  told  you  that  artificial  magnets, 
which  are  made  of  steel,  are  now  generally  used  in 
preference  to  the  real  magnet,  because  they  can  be 
procured  with  greater  ease,  may  be  varied  in  their 
form  more  easily,  and  will  communicate  the  magnetic 
virtue  more  powerfully. 

C.  How  are  they  made  ] 


424 


MAGNETISM. 


T.  The  best  method  of  making  artificial  magnets  is 
to  apply  one  or  more  powerful  magnets  to  pieces  of 
hard  steel,  taking  care  to  apply  the  north  pole  of  the 
magnet  or  magnets  to  that  extremity  of  the  steel  which 
is  required  to  be  made  the  south  pole,  and  to  apply  the 
south  pole  of  the  magnet  to  the  opposite  extremity  of 
the  piece  of  steel. 

J.  Has  a  magnet,  by  communicating  its  properties 
to  other  bodies,  its  own  power  diminished  1 

T.  No,  it  is  even  increased  by  it. — A  bar  of  iron 
three  or  four  feet  long,  kept  some  time  in  a  vertical 
position,  will  become  magnetic,  the  lower  extremity  of 
it  attracting  the  south  pole,  and  repelling  the  north 
pole.  But  if  the  bar  be  inverted,  the  polarity  will  be 
reversed. 

C.  Will  steel  produce  the  same  effects  ? 

T,  It  will  not ;  the  iron  must  be  soft,  and  hence 
bars  of  iron  that  have  been  long  in  a  perpendiculai 
position  are  generally  found  to  be  magnetical,  as  fire 
irons,  bars  of  windows,  &c. — If  a  long  piece  of  hard 
iron  be  made  red  hot,  and  then  left  to  cool  in  the 
direction  of  the  magnetical  line,  it  usually  becomes 
magnetical. 

Striking  an  iron  bar  with  a  hammer,  or  rubbing  it 
with  a  file,  while  held  in  this  direction,  renders  it  mag- 
netical. An  electric  shock,  and  lightning,  frequently 
render  iron  magnetic. 

J.  An  artificial  magnet,  you  say,  is  often  more 
powerful  than  the  real  one ;  can  a  magnet,  there- 
fore, communicate  to  steel  a  stronger  power  than  it 
possesses  1 

T.  Certainly  not:  but  two  or  more  magnets^  joined 
together,  may  communicate  a  greater  power  to  a 
piece  of  steel  than  either  of  them  possesses  singly. 

C.  Then  you  gain  power  according  to  the  number 
of  magnets  made  use  of? 

T.  Yes  ;  very  powerful  magnets  may  be  formed  by 
first  constructing  several  weak  magnets,  and  then 
joining  them  together  to  form  a  compound  one,  and  to 
act  more  powerfully  upon  a  piece  of  steel. 


EXPERIMENTS. 


425 


The  following  methods  are  among  the  best  for 
forming  artificial  magnets : 


Fig.  3. 


1.  Place  two  magnetic  bars  a  and  b  in  a  line,  so 
that  the  north  or  marked  end  of  one  shall  be  opposite 
to  the  south  end  of  the  other,  but  at  such  a  distance, 
that  the  magnet  c,  to  be  touched,  may  rest  with  its 
marked  end  on  the  unmarked  end  of  b,  and  its  un- 
marked end  on  the  marked  end  of  a.  Now  apply 
the  north  end  of  the  magnet  l,  and  the  south  end  of 
D,  to  the  middle  of  c,  the  opposite  ends  being  elevated 
as  in  the  figure.  Draw  l  and  d  asunder  along  the 
bar  c,  one  towards  a,  the  other  towards  b,  preserving 
the  same  elevation  :  remove  l  d  a  foot  or  more  from 
the  bar  when  they  are  off  the  ends,  then  bring  the 
north  and  south  poles  of  these  magnets  together,  and 
apply  them  again  to  the  middle  of  the  bar  c  as  before  : 
the  same  process  is  to  be  repeated  five  or  six  times, 
then  turn  the  bar,  and  touch  the  other  three  sides  in 
the  same  way,  and  with  care  the  bar  will  acquire  a 
strong  fixed  magnetism. 


fig.  4. 

2.  Upon  a  similar  principle,  two  bars,  a  b,  c  d,  may 
be  rendered  magnetic.    These  are  supported  by  two 


426 


MAGNETISM. 


bars  of  iron,  and  they  are  so  placed  that  the  marked 
end  B  may  be  opposite  to  the  unmarked  end  d  ;  then 
place  the  two  attracting  poles  g  i  on  the  middle  of 
A  B,  as  in  the  figure,  moving  them  slowly  over  it  ten 
or  fifteen  times.  The  same  operation  is  to  be  per- 
formed on  c  D,  having  first  changed  the  poles  of  the 
bars,  and  then  on  the  other  faces  of  the  bars ;  and 
the  business  is  accomplished. 

The  touch  thus  com- 
municated may  be  farther 
increased  by  rubbing  the 
different  faces  of  the  bars 
with  sets  of  magnetic 
bars,  disposed  thus : 

J.  I  suppose  all  the 
bars  should  be  very 
smooth.  Fig.  5. 

2\  Yes,  they  should  be 
well  polished,  the  sides  and  ends  made  quite  flat,  and 
the  angles  quite  square. 

There  are  many  magnets  made  in  the  shape  of  horse- 
shoes, these  are  called  horse-shoe  magnets,  and  they 
retain  their  power  very  long  by  taking  care  to  join  a 
piece  of  iron  to  the  end  as  soon  as  it  is  done  with. 

C.  Does  that  prevent  its  power  from  escaping  1 

T.  It  should  seem  so  ;  the  power  of  a  magnet  is 
even  increased  by  suffering  a  piece  of  iron  to  remain 
attached  to  one  or  both  of  its  poles.  Of  course  a 
single  magnet  should  always  be  thus  left. 

J,  How  is  magnetism  communicated  to  compass 
needles  1 

T,  Fasten  the  needle  down  on  a  board,  and  draw 
magnets  about  six  inches  long,  in  each  hand,  from  the 
centre  of  the  needle  outwards  ;  then  raise  the  bars  to 
a  considerable  distance  from  the  needle,  and  bring 
them  perpendicularly  down  on  its  centre,  and  draw 
them  over  again,  and  repeat  this  operation  about 
twenty  times,  and  the  ends  of  the  needle  will  point  to 
the  poles  contrary  to  those  that  touched  them. 


VARIATION  OF  THE  COMPASS.  427 
C.  1  remember  seeing  a  compass  when  I  was  on 
board  the  frigate  that  lay  off  Worthing :  the  needle 
was  in  a  box,  with  a  glass  over  it. 

T.  The  mariner's  compass  consists  of  the  box,  the 
card  or  fly,  and  the  needle.  The  box  is  circular, 
and  is  so  suspended  as  to  retain  its  horizontal  position 
in  all  the  motions  of  the  ship.  The  glass  is  intended 
to  prevent  any  motion  of  the  card  by  the  wind;  the 
card  or  fly  moves  with  the  needle,  which  is  very 
nicely  balanced  on  a  centre.  It  may,  however,  be 
noticed,  that  a  needle,  which  is  accurately  balanced 
before  it  is  magnetized,  will  lose  its  balance  by  being 
magnetized,  on  account  of  what  is  called  the  dipping, 
therefore  a  small  weight,  or  moveable  piece  of  brass, 
is  placed  on  one  side  of  the  needle,  by  the  shifting 
of  which  the  needle  will  always  be  balanced. 


CONVERSATION  IV. 

OF  THE  VARIATION  OF  THE  COMPASS. 

C.  You  said,  I  think,  that  the  magnet  pointed 
nearly  north  and  south ;  how  much  does  it  differ  from 
that  line  1 

T.  It  rarely  points  exactly  north  and  south,  and 
the  deviation  from  that  line  is  called  the  variation  of 
the  compass,  which  is  said  to  be  east  or  west. 

/.  Does  this  differ  at  different  times  ? 

r.  It  does ;  and  the  variation  is  very  different  in 
different  parts  of  the  world.  The  variation  is  not  the 
same  now  that  it  was  half  a  century  ago,  nor  is  it  the 
same  now  at  London  that  it  is  at  Bengal  or  Kamt- 
schatka.  The  needle  is  continually  traversing  slowly 
towards  the  east  and  west.  ° 

This  subject  was  first  attended  to  by  Mr.  Burrowes, 
about  the  year  1580,  and  he  found  the  variation  then, 
at  London,  about  11°  11'  east.  In  the  year  1657, 
the  needle  pointed  due  north  and  south  :  since  which 
the  variation  has  been  gradually  increasing  towards 
the  west;  and  in  the  year  1803  it  was  equal  to  some- 


428  MAGNETISM. 

thing  more  than  24"  west,  and  was  then  advancing 

towards  the  same  quarter. 

C.  That  is  at  the  rate  of  something  more  than  ten 
minutes  each  year  ? 

T.  It  is  ;  but  the  annual  variation  is  not  regular  ; 
it  is  more  one  year  than  another.  It  is  different  in 
the  several  months,  and  even  in  the  hours  of  the  day. 
Its  present  mean  variation  at  London  is  about  24^  33' 
west. 

J.  Then  if  I  want  to  set  a  globe  due  north  and 
south,  to  point  out  the  stars  by,  I  must  move  it 
about,  till  the  needle  in  the  compass  points  to  24^  33' 
west  ? 

r.  Just  so ;  and  mariners  knowing  this,  are  as 
well  able  to  sail  by  the  compass  as  if  it  pointed  due 
north. 

C.  You  mentioned  the  property  which  the  needle 
had  0^  dipping,  after  the  magnetic  fluid  was  communi- 
cated to  it :  is  that  always  the  same  1 

T.  It  varies  slightly.  It  was  discovered  by 
llobert  Norman,  a  compass-maker,  in  the  year  1576, 
and  he  then  found  it  to  dip  nearly  72«,  and  from 
many  observations  made  at  the  Royal  Society,  it  is 
found  to  be  now  about  70°  32'. 

/.  Does  it  differ  in  different  places  ? 
T.  Yes:  in  the  year  1773  observations  were  made 
on  the  subject  in  a  voyage  towards  the  north  pole,  and 
from  these  it  appears  that 

In  latitude  60O    18'  the  dip  was  75o  0' 

  70     45   —  77  52 

 —  80      12  •   81  52 

  80      27    82  2i 

The  dip  always  increasing  as  the  latitude  is  greater. 

I  will  shew  you  an  experiment  on  this  subject. 
Here  is  a  magnetic  bar,  and  a  small  dipping  needle  ; 
if  I  carry  the  needle,  suspended  freely  on  a  pivot, 
from  one  end  of  the  magnetic  bar  to  the  other,  it  will, 
when  directly  over  the  south  pole,  settle  directly  per- 
pendicularly to  it,  the  north  end  being  next  to  the 


SUMMARY,  429 
south  pole :  as  the  needle  is  moved,  the  dip  grows 
less  and  less,  and  when  it  comes  to  the  magnetic 
centre  it  will  be  parallel  to  the  bar  ;  afterwards  the 
south  end  of  the  needle  will  dip,  and  when  it  comes 
directly  over  the  north  pole,  it  will  be  again  perpen- 
dicular to  the  bar. 

The  following  facts  are  deserving  of  recollection. 

1.  Iron  is  the  only  body  capable  of  being  affected 
by  magnetism. 

2.  Every  magnet  has  two  opposite  points,  called 
poles, 

3.  A  magnet  freely  suspended  arranges  itself  so  that 
these  poles  point  nearly  north  and  south.  This  is 
called  the  directive  property,  or  polarity,  of  the 
magnet. 

4.  When  two  magnets  approach  each  other,  the 
poles  of  the  same  names,  that  is,  both  north,  or  both 
south,  repel  each  other. 

5.  Poles  of  different  names  attract  each  other. 

6.  The  loadstone  is  an  iron  ore  naturally  possessing 
magnetism. 

7.  Magnetism  may  be  communicated  to  iron  and 
steel, 

8.  A  steel  needle  rendered  magnetic,  and  fitted  up 
in  a  box,  so  as  to  move  freely  in  any  direction,  con- 
stitutes the  mariners*  compass. 

C.  I  think  there  is  a  similarity  between  electricity 
and  magnetism. 

T.  You  are  right ;  there  is  a  considerable  analogy, 
and  a  remarkable  difference,  also,  between  magnetism 
and  electricity. 

Electricity  is  of  two  sorts,  positive  and  negative  ; 
bodies  possessed  of  the  same  sort  of  electricity  repel 
each  other;  and  those  possessed  of  different  sorts 
attract  each  other. — In  Magnetism,  every  magnet 
has  two  poles ;  poles  of  the  same  name  repel  each 
other,  and  the  contrary  poles  attract  each  other. 
^  In  Electricity,  when  a  body,  in  its  natural  state, 
is  brought  near  to  one  that  is  electrified,  it  acquires  a 
contrary  electricity,  and  becomes  attracted  by  it.— In 


430  MAGNETISM. 
MAGNETisM,*when  an  iron  substance  is  brought  near 
one  pole  of  a  magnet,  it  acquires  a  contrary  polarity, 
and  becomes  attracted  by  it. 

One  sort  of  electricity  cannot  be  produced  by  itself. 
In  like  manner,  no  body  can  have  only  one  magnetic 
pole. 

The  electric  virtue  may  be  retained  by  electrics, 
but  it  pervades  conducting  substances.  The  magnetic 
virtue  is  retained  by  iron,  but  it  pervades  all  other 
bodies. 

On  the  contrary  :  The  magnetic  povv^er  differs  from 
the  electric,  as  it  does  not  affect  the  senses  with  light, 
smell,  taste,  or  noise,  as  the  electric  does. 

Magnets  attract  only  iron,  but  the  electric  fluid 
attracts  bodies  of  every  sort. 

The  electric  virtue  resides  on  the  surface  of  electri- 
fied bodies,  but  the  magnetic  is  internal. 

A  magnet  loses  nothing  of  its  power  by  magnifying 
bodies,  but  an  electrified  body  loses  part  of  its  elec- 
tricity by  electrifying  other  bodies. 


ELECTRICITY. 


CONVERSATION  I. 


INTRODUCTION. 

THE  EARLY   HISTORY   OF  ELECTRICITY. 
TUTOR  CHARLES  JAMES. 

Tutor.  If  I  rub  pretty  briskly  with  my  hand  this 
stick  of  sealing  wax,  and  then  hold  it  near  any  small 
light  substances,  as  little  pieces  of  paper,  the  wax 
will  attract  them  3  that  is,  if  the  wax  be  held  within 
an  inch  or  more  of  the  paper,  they  will  jump  up  and 
adhere  to  it. 

Charles.  They  do  ;  and  I  think  I  have  heard  you 
call  this  the  effects  of  electricity,  but  1  do  not  know 
what  electricity  is. 

T,  It  is  the  case  with  this  part  of  science  as  with 
many  others,  we  know  it  only  by  the  effects  which  it 
produces.  As  I  have  not  hitherto,  in  these  Conver- 
sations, attempted  to  bewilder  your  minds  with  useless 
theories,  neither  shall  I,  m  the  present  case,  attempt 
to  say  what  the  electrical  fluid  is :  its  action  is  well 
known  ;  it  seems  diffused  over  every  portion  of  matter 
with  which  we  are  acquainted,  and,  by  the  use  of 
proper  methods,  it  is  as  easily  collected  from  sur- 
rounding bodies  as  water  is  taken  from  a  river. 

James.  I  see  no  fluid  attaching  to  the  sealing  wax 
when  you  have  rubbed  it. 

T.  You  do  not  see  the  air  which  you  breathe,  and 
with  which  you  are  surrounded,  yet  we  have  shewn 
you  *  that  it  is  a  fluid,  and  may  be  taken  from  any 
vessel,  as  certainly,  though  not  with  so  much  ease,  as 

*  See  Conversations  on  Pneumatics. 


432  ELECTRICITY, 
water  may  be  poured  from  this  glass.  With  the  ex- 
ercise of  a  small  degree  of  patience,  you  shall  set 
such  experiments  as  will  not  fail  to  convince  you  that 
there  is  as  certainly  a  fluid,  which  is  called  the  elec- 
tric fluid,  as  there  are  such  fluids  as  water  and  air. 

C.  Water  must  have  been  known  since  the  crea- 
tion,' and  the  existence  of  the  air  could  not  long  re- 
main a  secret,  but  who  discovered  the  electric  fluid, 
which  is  not  at  all  evident  to  the  sense  either  of  sight 
or  feeling  7  •     ,  r 

T,  Thales,  who  lived  six  centuries  betore  the 
Christian  era,  was  the  first  who  observed  the  electri- 
cal properties  of  amber,  and  he  was  so  struck  with  the 
appearances,  that  he  supposed  it  to  be  animated. 

J.  Does  amber  attract  light  bodies,  like  sealing 
wax  1 

T.  Yes,  it  does  ;  and  there  are  many  other  sub- 
stances, as  well  as  these,  that  have  the  same  power. 
After  Thales,  the  first  person  we  read  of  that  noticed 
this  subject  was  Theophrastus,  who  discovered  that 
iourmalin  has  the  power  of  attracting  light  bodies. 
It  does  not,  however,  appear  that  the  subject,  though 
very  curious,  excited  much  attention  until  about  the 
year  1600,  when  Dr,  Gilbert,  an  English  physician, 
examined  a  great  variety  of  substances,  with  a  view 
of  ascertaining  how  far  they  might  or  might  not  be 
ranked  among  electrics. 

C.  What  is  meant  by  an  electric  7 

T.  Any  substance  which  being  excited  or  rubbed 
by  the  hand,  or  by  a  woollen  cloth,  or  other  means, 
and  has  the  power  of  attracting  light  bodies,  is  called 
an  electric. 

/.  Is  not  electricity  accompanied  with  a  pecuhar 
kind  of  light,  and  with  sparks  1 

T.  It  is,  of  which  we  shall  speak  more  at  large 
hereafter  :  the  celebrated  iMr.  Boyle  is  supposed  to 
have  been  one  of  the  first  persons  who  got  a  glimpse 
of  the  electrical  light,  or  who  seems  to  have  noticed 
it  by  rubbing  a  diamond  in  the  dark.  But  he  little 
imagined,  at  that  time^  what  astonishing  effects  would 


ELECTRIC  FLUID.  43^3 
afterwards  be  produced  by  the  same  power.  Sir 
Isaac  Newton  was  the  first  who  observed  that  excited 
glass  attracted  light  bodies  on  the  side  opposite  to  that 
on  which  it  was  rubbed. 

C.  How  did  he  make  the  discovery? 
T.  Having  laid  upon  the  table  a  round  piece  of 
glass,  about  two  inches  broad,  in  a  brass  ring,  by 
which  it  was  raised  from  the  table  about  the  eighth  of 
an  inch,  and  then  rubbing  the  glass,  some  little  bits  of 
paper  .which  were  under  it  were  attracted  by  it,  and 
moved  very  nimbly  to  and  from  the  glass. 

C.  I  remember  standing  by  a  glazier  when  he  was 
cementing,  that  is,  rubbing  over  some  window-lights 
with  oil,  and  cleaning  it  off  with  a  stiff  brush  and 
whitmg,  and  the  little  pieces  of  whiting  under  the 
glass  kept  continually  leaping  up  and  down,  as  the 
brush  moved  over  the  glass. 

T,  That  was,  undoubtedly,  an  electrical  appear- 
ance, but  I  do  not  remember  having  ever  seen  it 
noticed  by  any  writer  on  electricity.  A  complete 
history  of  this  science  is  given  by  Dr.  Priestley,  which 
will,  hereafter,  afford  you  much  entertainment  and 
interesting  instruction.  To-morrow  we  shall  enter 
into  the  practical  part  of  the  subject ;  and  I  doubt 
not  that  the  experiments  in  this  part  of  science  will 
be  as  interesting  as  those  in  any  other  which  you 
have  been  studying.  The  electric  light,  exhibited  in 
different  forms ;  the  various  signs  of  attraction  and 
repulsion  acting  on  all  bodies ;  the  electric  shock, 
and  the  explosion  of  the  battery,  will  give  you  plea- 
sure, and  excite  your  admiration. 


CONVERSATION  II. 

OF  ELECTRIC  ATTRACTION  AND   REPULSION  OF  ELEC- 
TRICS AND  CONDUCTORS, 

^  T.  You  must  for  a  little  time,  that  is,  till  we  ex» 
hibit  before  you  experiments  to  prove  it,  take  it  for 
granted,  that  the  earth  and  all  bodies  with  which  we 
U 


434  ELECTRICITY, 
are  acquainted,  contain  a  certain  quantity  of  exceed- 
ingly elastic  and  penetrating  fluid,  which  philosophers 
call  the  electric  fluid. 

C.  You  say  a  certain  quantity  ;  is  it  limited  ? 

T,  Like  other  bodies  it  undoubtedly  has  its  limits  ; 
this  glass  will  hold  a  certain  quantity  of  water,  but  if 
1  attempt  to  pour  into  it  more  than  that  quantity,  a 
part  will  flow  over.  So  it  is  with  the  electric  fluid  : 
there  is  a  certain  quantity  which  belongs  to  all  bodies, 
and  this  is  called  their  natural  quantity,  and  so  long 
as  a  body  contains  neither  more  nor  less  than  this 
quantity,  no  sensible  eflect  is  produced. 

J.  Has  this  table  electricity  in  it  7 

T.  Yes,  and  so  has  the  inkstand,  and  every  thmg 
else  in  the  room  ;  and  if  I  were  to  take  proper  means 
to  put  more  into  it  than  it  now  has,  and  you  were  to 
put  your  knuckle  to  it,  it  would  throw  it  out  in  the 
shape  of  sparks. 

J.  I  should  like  to  see  this  done. 

C.  But  what  would  happen  if  you  should  take  away 
some  of  its  natural  quantity  1 

T.  Why  then  if  you  presented  any  part  of  your 
body  to  the  table,  as  your  knuckle,  a  spark  would  go 
from  you  to  the  table. 

J.  But  perhaps  Charles  might  not  have  more 
than  his  natural  share,  and  in  that  case  he  could 
not  spare  any. 

T.  True  5  but  to  provide  for  this,  the  earth  on 
which  he  stands  would  lend  him  a  little,  to  make  up 
for  what  he  parted  with  to  the  table.         •     j  i,  ii 

J.  This  must  be  an  amusing  study  ;  I  thmtc  I  shall 
like  it  better  than  any  of  the  others. 

T.  Take  care  that  you  do  not  pay  for  the  amuse- 
ment before  we  have  done. 

Here  is  a  glass  tube  about  eighteen  inches  long, 
and  perhaps  an  inch  or  more  in  diameter  ;  I  rub  it  up 
and  down  quickly  in  my  hand,  which  is  dry  and 
warm,  and  now  I  will  present  it  to  these  fragments  of 
paper,  thread,  and  gold-leaf:  you  see  they  all  move 
to  it.    That  is  called  electrical  attraction. 


ELECTRICS  AND  CONDUCTOIIS.  435 

C.  They  jump  back  again  now  j  and  now  thev  re- 
turn to  the  glass.  ^ 

r.  They  are,  in  fact,  alternately  attracted  and  re- 
pelled, and  this  will  last  several  minutes  if  the  dass 
be  strongly  excited.  I  will  rub  it  again  ;  present  your 
knuckle  to  it  in  several  parts  one  after  another. 

/.  What  is  that  snapping?  I  feel,  likewise,  some- 
thing like  the  pricking  of  a  pin. 

The  snapping  is  occasioned  by  little  sparks 
which  come  from  the  tube  to  your  knuckle,  and  these 
give  the  sensation  of  pain. 

ment  ^         ^^^^  ^  ^^^^  ^^^^^^  experi- 

C  The  sparks  are  evident  enough  now,  but  I  do 
not  know  where  they  can  come  from. 

J.  The  air  and  every  thing  is  full  of  the  fluid 
which  appears  in  the  shape  of  sparks  ;  and  whatever 
be^  he  cause  which  I  do  not  attempt  to  explain,  the 
rubbing  of  the  glass  with  the  hand  collects  it  from 
the  air,  and  having  now  more  than  its  natural  share. 
It  parts  with  It  to  you,  or  to  me,  or  to  any  body  else 
that  may  be  near  enough  to  receive  it. 

substance  besides  the  hand  ex- 

cite  the  tube  7 

nrJ*  lT^'^^^''^^?^^''^  ^""^  t^^^s  science, 

are  called  the  rubbers  ;  and  the  glass  tube,  or  what- 

dectric  ^^'""^  ^^""^  ^^""'^^^^  '^^"^'^ 

being  4dtedV'"  '''''     ""^^  substances  capable  of 

T.  You  may  rub  this  poker,  or  the  round  ruler, 
lor  ever,  without  obtaining  an  electric  spark  from  it. 

J.  J3ut  you  said  one  might  get  a  spark  from  the 
mahogany  table  if  it  had  more  than  its  share. 

1  •/  ^"^^  ""^^  ^^^^  sparks  from  the  poker 
o^f  thtdecttZ^^^^^  "^^^        ^^^^^  commonLare 

P.rfL^'"'^^'!?  you  distinguish  between  bodies  that 
can  be,  and  those  that  cannot  be,  excited  ? 


436  ELECTRICITY. 

r.  The  former,  as  I  have  told  you,  are  called  dec- 
tries,  as  the  glass  tube  :  the  latter,  such  as  the  poker, 
the  ruler,  your  body,  and  a  thousand  other  substances 
are  denominated  condyle  tors. 

C.  I  should  be  glad  to  know  the  reason  of  the  dis- 
tinction, because  I  shall  be  more  likely  to  remem- 
ber it. 

T.  That  is  right:  when  you  held  your  knuckle 
to  the  glass  tube,  you  had  several  sparks  from  the 
different  parts  of  it :  but  if  I,  by  any  means,  over- 
charged a  conductor,  as  this  poker,  all  the  electri- 
city will  come  away  at  a  single  spark,  because  the 
superabundant  quantity  flows  instantaneously  from 
every  part  to  that  point  where  it  has  an  opportunity 
of  getting  out.  I  will  illustrate  this  by  an  experiment. 
But  first  of  all  let  me  tell  you,  that  all  electrics  are 
called  also  non-conductors, 

J.  Do  you  call  the  glass  tube  a  non-conductor,  be- 
cause it  does  not  suffer  the  electric  fluid  to  pass  from 
one  part  of  it  to  another  1 

T,  I  do :  silk,  if  dry,  is  a  non-conductor.  With 
this  skein  of  sewing  silk  I 
hang  the  poker,  or  other  me- 
tal substance,  a,  to  a  hook  in 
the  ceiling,  so  as  to  be  about 
twelve  inches  from  it ;  under- 
neath, and  near  the  extremity, 
are  some  small  substances,  as 
bits  of  paper,  &c.  I  will  ex- 
cite the  glass  tube,  and  present 
it  to  the  upper  part  of  the  po- 
ker. 

C.  They  are  all  attracted; 
but  now  you  take  away  the 
glass  they  are  quiet. 

T.  It  is  evident  that  the  elec- 
tric fluid  passed  from  one  part 
of  the  tube  through  the  poker, 
which  is  a  conductor,  to  the  Fig.  1. 


ELECTRICS  AND  CONDUCTORS.  437 
paper,  and  attracted  it:  — if  the  glass  be  properly 
excited  you  may  take  sparks  from  the  poker. 

Would  not  the  same  happen  if  another  glass 
tube  were  placed  in  the  stead  of  the  poker  ] 

T,  You  shall  try.—- now  I  have  put  the  glass  in  the 
place  of  the  poker,  but  let  me  excite  the  other  tube  as 
much  as  I  will,  no  effect  can  be  produced  on  the 
paper  .--—there  are  no  signs  of  electrical  attraction, 
which  shews  that  the  electric  fluid  will  not  pass 
through  glass. 

C.  What  would  have  happened  if  any  conducting- 
substance  had  been  used,  instead  of  silk,  to  suspend 
the  iron  poker  1 

r.  If  I  had  suspended  the  poker  with  a  moistened 
hempen  string,  the  electric  fluid  would  have  all  passed 
away  through  that,  and  there  would  have  been  no  (or 
very  trifling)  appearance  of  electricity  at  the  end  of 
the  poker. 

You  may  vary  these  experiments  till  you  make 
yourselves  perfect  with  regard  to  the  distinction  be- 
tween electrics  and  conductors.  Sealing-wax  is  an 
electric,  and  may  be  excited  as  well  as  a  glass  tube 
and  will  produce  similar  effects.  I  will  give  you  a 
list  of  electrics,  and  another  of  conductors,  disposed 
according  to  the  order  of  their  perfection,  beginning  in 
each  list  with  the  most  perfect  of  their  class  ;  thus 
glass  is  a  better  electric  than  amber ;  and  gold  a  bet- 
ter conductor  than  silver. 

TABLE. 

Electrics.  Conductors. 

Glass  of  all  kinds.  All  the  metals  in  the  fol- 

All  precious  stones,  the  most  lowing  order  :~ 

transparent  the  best.  Gold  ;  silver ;  copper  ;  pla- 

T^^^"-''  tina;  brass;  iron;  tin; 

^"^P^"^-  quicksilver;  lead. 

All  resinous  substances.  The  semi-metals.* 

Wax  of  all  kinds.  Metallic  ores.* 

Silk  and  cotton.  Charcoal. 


Electrics. 

Dry  external  substances, 
as  feathers,  wool,  and 
hair. 

Paper  ;  loaf  sugar. 
Air,  when  quite  dry. 
Oils  and  metallic  oxides.  * 
Ashes  of  animal  and  vege- 
table substances. 
Most  hard  stones. 


ELECTRICITY. 

Conductors. 
The  fluids  of  an  animal 
body. 

Water,  especially  salt  wa- 
ter, and  other  fluids,  ex- 
cept oil. 
Ice,  snow. 

Most  saline  substances. 
Earthy  substances. 
Smoke  ;  steam,  and  even 
A  vacuum. 


CONVEUSATION  III. 

OF  TPIE  ELECTRICAL  MACHINE. 

T.  I  will  now  explain  to  you  the  construction  of 
the  electrical  machine,  and  shew  you  how  to  use  it. 

C.  For  what  purpose  is  it  used  1 

r.  Soon  after  the  subject  of  the  electric  fluid  en- 
gaged the  attention  of  men  of  science,  they  began  to 
contrive  the  readiest  methods  of  collecting  large  quan- 
tities of  it.  By  rubbing  this  stick  of  sealing-wax  I 
can  collect  a  small  portion  ;  if  I  excite  or  rub  the 
glass  tube,  I  get  still  more.  The  object,  therefore, 
was,  to  find  out  a  machine  by  which  the  largest  quan- 
tities can  be  collected,  with  as  little  trouble  and  ex- 
pense as  may  be. 

J.  You  get  more  electricity  from  the  tube  than 
from  the  sealing-wax,  because  it  is  five  or  six  times  as 
large  ;  by  increasing  the  size  of  the  tube  you  would 
increase  the  quantity  of  the  electric  fluid,  I  should 
think. 

T.  That  is  a  natural  conclusion.    But  if  you  look 


*  This  and  other  chemical  terms  are  explained  and 
familiarly  illustrated  in  a  work,  by  the  author  of  the 
Scientilic  Dialogues,  entitled  "  Dialogues  on  Chemis- 
try," &c. 


ELECTRICAL  MACHINE.  439 
to  the  table  of  electrics  which  I  made  out  yesterday, 
you  will  see,  that  had  the  wax  been  as  large  as  the 
glass  tube,  it  would  not  have  collected  so  much  of 
the  electric  fluid,  because,  in  its  own  nature,  it  is  not 
so  good  an  electric. 

C.  In  that  table  glass  stands  as  the  most  perfect 
electric,  but  there  are  several  substances  between  it 
and  wax,  all  of  which  are,  I  believe,  more  perfect 
electrics  than  wax. 

r.  They  are ;  electricians,  therefore,  had  no  hesi- 
tation as  to  the  nature  of  the  substance :  they  fixed 
on  glass,  which,  being  easily  melted  and  run,  or  blown 
mto  all  sorts  of  forms,  is,  on  that  account,  very 
valuable. 

The  most  common  form  that  is  now  used  is  that  of 
a  glass  cylinder,  from  five  or  six  inches  in  diameter  to 
ten  or  twelve.    Here  is  one  completely  fitted  up. 


Fig.  2. 


The  cylinder  a  b  is  about  eight  inches  in  diameter, 
and  twelve  in  length  ;  this  I  turn  round  in  the  frame- 
work with  the  handle  dc. 

J.  What  is  the  piece  of  black  silk  k  for  ? 

r.  The  cylinder  would  be  of  no  use  without  a  rub- 
ber you  know :  on  which  account  you  see  the  glass 
pillar  R  s,  which,  being  cemented  into  a  piece  of  hard 


440 


ELECTRICITY. 


wood,  is  made  to  screw  into  the  bottom  of  the  ma- 
chine ;  on  the  pillar  is  a  cushion  to  which  is  attached 
the  piece  of  black  silk. 

C.  And  I  perceive  the  cushion  is  made  to  press 
very  hard  against  the  glass. 

T.  This  pressure,  when  the  cylinder  is  turned  round 
fast,  acts  precisely  like  the  rubbing  of  the  tube  by 
the  hand,  though  in  a  still  more  perfect  manner.  I 
will  turn  it  round. 

J.  Here  is  not  much  sign  of  electricity  yet. 

T.  No  :  the  machine  is  complete,  but  it  has  no 
means  of  collecting  the  fluid  from  the  surrounding 
bodies  :  for  you  see  the  cushion  or  rubber  is  fixed  on 
a  glass  pillar,  and  glass  will  not  conduct  the  electric 
fluid. 

C.  Nevertheless  it  does,  by  turning  round,  shew 
some  signs  of  attraction. 

T.  Every  body  in  nature  with  which  we  are  ac- 
quainted possesses  a  portion  of  this  fluid,  and  there- 
fore the  signs  which  are  now  evident  arise  from  the 
small  quantity  which  exists  in  the  rubber  itself,  and 
the  atmosphere  that  immediately  surrounds  the  ma- 
chine. 

C.  Would  the  case  be  different  if  the  rubber  were 
fixed  on  a  conducting  substance  instead  of  glass  1 

T.  It  would  ;  but  there  is  a  much  easier  method  ; 
I  will  hang  on  this  brass  chain  to  the  cushion  at  r, 
which  being  several  feet  long  lies  on  the  table,  or  on 
the  floor,  and  this  you  know  is  connected,  by  means 
of  other  objects,  with  the  earth,  which  is  the  grand 
reservoir  of  the  electric  fluid.  Now  see  the  effect  of 
turning  round  the  cylinder :  but  I  must  make  every 
part  of  it  dry  and  rather  warm,  by  rubbing  it  with  a 
dry  warm  cloth. 

J.  It  is  indeed  very  powerful.  What  a  crackling 
noise  it  makes  ! 

T,  Shut  the  window-shutters. 

C.  The  appearance  is  very  beautiful  j  the  flashes 
from  the  silk  dart  all  round  the  cylinder. 

T.  I  will  now  bring  to  the  cylinder  the  tin  con- 


ELECTRICAL  MACHINE.  441 
ductor  L,  which  is  also  placed  on  a  glass  pillar  fn 
fixed  in  the  stand  at  f.  r  > 

/.  What  are  the  points  in  the  tin  conductor  for  ? 
T.  They  are  intended  to  collect  the  fluid  from  the 
cylinder ;  I  will  turn  the  cylinder,  and  do  you  hold 
your  knuckle  within  four  or  five  inches  of  the  con- 
ductor. 

C.  The  painful  sensations  which  these  sparks  oc- 
casion, prove  that  the  electric  fluid  is  a  very  powerful 
agent  when  collected  in  large  quantities. 

T.  To  shew  you  the  nature  of  conducting  bodies, 
1  will  now  throw  another  brass  chain  over  the  con- 
ductor, so  that  one  end  of  it  may  lie  on  the  floor :  see 
now  if  you  can  get  any  sparks  while  I  turn  the  ma- 
chine. 

^  /.  No,  I  can  get  none,  put  my  knuckle  as  near  to 
It  as  I  will  :— does  it  all  run  away  by  the  chain  1 

It  does  :  a  piece  of  brass  or  iron  wire  would  do 
as  well;  and  so  would  any  conducting  substance 
which  touched  the  conductor  with  one  end,  and  the 
floor  with  the  other  :  your  body  would  do  as  well  as 
the  chain.  Place  your  hand  on  the  conductor  while 
I  turn  round  the  cylinder:  and  let- your  brother 
bring  his  knuckle  near  the  conductor. 
C.  I  can  get  no  spark. 

T  It  runs  through  James  to  the  earth,  and  you 
see  his  body  is  a  conductor  as  well  as  the  chain. 
With  a  very  little  contrivance  I  can  take  sparks 
trom  you  or  James,  as  well  as  you  did  from  the  con- 
ductor. 

/ .  I  should  like  to  see  how  that  is  done. 

T.  Here  is  a  small  stool,  having  a  mahogany  top 
and  glass  legs.  If  you  stand  on  that,  and  put  your 
hand  on  the  conductor,  the  electricity  will  pass  from 
the  conductor  to  your  body. 

C.  Will  the  glass  legs  prevent  it  from  running 
from  him  to  the  earth  ? 

T,  They  will :  and  therefore  what  he  receives  from 
the  conductor,  he  will  be  ready  to  part  with  to  any 
U2  ^ 


442  ELECTRICITY. 

of  the  surrounding  bodies,  or  to  you  if  you  bring  youi 
hand  near  enough  to  any  part  of  him. 

J.  The  sparks  are  more  painful  in  coming  through 
my  clothes  than  when  I  received  them  on  my  bare 
hand. 

T,  You  understand,  I  hope,  the  process. 

C.  By  means  of  the  chain  trailing  on  the  ground, 
the  electric  fluid  is  collected  from  the  earth  on  the 
glass  cylinder,  which  gives  it  through  the  points  to 
the  conductor  ;  from  this  it  may  be  conveyed  away 
again  by  means  of  other  conductors. 

T.  Whatever  body  is  supported,  or  prevented  from 
touching  the  earth,  or  communicating  with  it,  by 
means  of  glass  or  other  non-conducting  substances, 
is  said  to  be  insulated.  Thus  a  body  suspended  on 
a  silk  line  is  insulated,  and  so  is  any  substance  that 
stands  on  glass,  or  resin,  or  wax,  provided  that  these 
are  in  a  dry  state,  for  moisture  will  conduct  away 
the  electric  fluid  from  any  charged  body. 

CONVERSATION  IV. 

OF  THE  ELECTRICAL  MACHINE. 

C.  What  is  that  shining  stuff"  which  I  saw  you  put 
on  the  rubber  yesterday  ?  , 

T.  It  is  called  amalgam ;  the  rubber,  by  itself, 
would  produce  a  very  slight  excitation ;  but  its 
power  is  greatly  increased  by  laying  upon  it  a  little 
of  this  amalgam,  which  is  made  of  quicksilver,  zinc, 
and  tin-foil,  with  a  little  tallow  or  mutton  suet. 

J.  Is  there  any  art  required  in  using  this  amal- 
gam 1 

T.  When  the  rubber  and  silk  flap  are  very  clean 
and  dry,  and  in  their  place,  then  spread  a  little  of  the 
amalgam  upon  a  piece  of  leather,  and  apply  it  to  the 
under  part  of  the  glass  cylinder,  while  it  is  revolving 
from  you  ;  by  this  means  particles  of  the  amalgam 
will  be  carried  by  the  glass  itself  to  the  lower  part  of 
the  rubber,  and  will  increase  the  excitation. 


ELECTHICAL  MACHINE.  443 

C.  I  think  I  once  saw  a  globe  instead  of  a  cylinder 
for  an  electrical  machine. 

T.  You  might :  globes  were  used  before  cylinders, 
but  the  latter  are  the  most  convenient  of  the  two! 
The  most  powerful  electrical  machines  are  fitted  with 
flat  plates  of  glass.  In  our  experiments  we  shall  be 
content  with  the  cylinder,  which  will  answer  every 
purpose  of  explaining  the  principles  of  the  science. 

/.  As  I  was  able  to  conduct  the  electricity  from 
the  tm  conductor  to  the  ground,  could  I  likewise  act 
the  part  of  the  chain  by  conducting  the  fluid  from  the 
earth  to  the  cushion  1 

T,  Undoubtedly  :  I  will  take  ofF  the  chain,  and 
now  do  you  keep  your  hand  on  the  cushion  while  I 
turn  the  handle. 

/.  I  see  the  machine  works  as  well  as  when  the 
chain  was  on  the  ground. 

T,  Keep  your  present  position,  but  stand  on  the 
stool  with  glass  legs  ;  by  which  means  there  is  now 
all  communication  cut  ofF  between  the  cushion  and 
the  earth  ;  in  other  words,  the  cushion  is  completely 
insulated,  and  can  only  take  from  you  what  electricity 
It  can  get  from  your  body.  Go,  Charles,  and  shake 
hands  with  your  brother. 

C.  It  does  not  appear  that  the  machine  had  taken 
all  the  electricity  from  him,  for  he  gave  me  a  smart 
spark. 

T.  You  are  mistaken;  he  gave  you  nothing, but  he 
took  a  spark  from  you. 

C.  I  stood  on  the  ground,  I  was  not  electrified  • 
how  then  could  I  give  him  a  spark  ?  ' 

r.  The  machine  had  taken  from  James  the  electri- 
city that  was  in  his  body,  and  by  standing  on  the  stool, 
that  IS,  by  being  insulated,  he  had  no  means  of  re- 
ceiving any  more  from  the  earth,  or  any  surrounding 
objects;  the  moment,  therefore,  you  brought  your 
hand  near  him,  the  electricity  passed  from  you  to 
him. 

C.  I  certainly  felt  the  spark,  but  whether  it  went 


444 


ELECTRICITY. 


out  of,  or  entered  into,  my  hand,  I  cannot  tell ; 
have  I  then  less  than  my  share  now  ? 

r.  No  :  what  you  gave  to  your  brother  was  sup- 
plied immediately  from  the  earth.  Here  is  another 
glass-legged  stool ;  do  you  stand  on  this,  but  at  the 
distance  of  a  foot  or  two  from  your  brother,  who  still 
keeps  his  place.  I  take  the  electricity  from  him  by 
turning  the  machine,  and,  as  he  stands  on  the  stool, 
he  has  now  less  than  his  share.  But  you  have 
your  natural  share,  because,  though  you  also  are  insu- 
lated, yet  you  are  out  of  the  influence  of  the  machine ; 
extend,  therefore,  your  hand,  and  give  him  a  part  of 
the  electric  fluid  that  is  in  you. 

C.  I  have  given  him  a  spark. 

T.  And,  being  yourself  insulated,  you  have  now 
less  than  your  natural  quantity,  to  supply  which  you 
shall  have  some  from  me  :  give  me  your  hand.  Why 
you  draw  it  back  without  my  touching  it ! 

C.  I  did,  but  it  was  near  enough  to  get  a  strong 
spark  from  you. 

T,  When  a  person  has  less  electricity  than  his 
natural  share,  he  is  said  to  be  electrified  minus,  or  ne- 
gatively :  but  if  he  has  more  than  his  natural  share, 
he  is  said  to  be  electrified  plus,  or  positively. 

J.  Then  before  Charles  gave  me  the  spark  I  was 
electrified  minus,  and  when  he  had  given  it  me  he 
was  minus  till  he  received  it  from  you. 

T.  That  is  right.  Suppose  you  stand  on  a  stool 
and  hold  the  rubber,  and  Charles  stand  on  another 
stool,  and  touch  the  prime  conductor  l  while  I  turn 
the  machine,  which  of  you  will  be  plus,  and  which 
minus  electrified? 

/.  I  shall  be  minus,  because  I  give  to  the  rubber  : 
and  Charles  will  be  plus,  because  he  receives  from  the 
conductor  what  I  gave  to  the  rubber,  and  which  is 
carried  by  the  cylinder  to  the  conductor. 

T.  You  then  have  less  than  your  share,  and  your 
brother  has  more  than  he  ought  to  have.  Now  if  I 
get   another  glass-legged  stool,  I  can  take  from 


ELECTRICAL  MACHINE.  443 
Charles  what  he  has  too  much,  and  give  it  to  you 
who  have  too  little. 

C.  Is  it  necessary  that  you  should  be  insulated  for 
this  purpose  ? 

T,  By  being  insulated  I  may  perhaps  carry  back 
to  J ames  the  very  electricity  which  passed  from  him 
to  you.  But  if  I  stand  on  the  ground,  the  quantity 
which  I  take  from  you  will  pass  into  the  earth,  be- 
cause I  cannot,  unless  I  am  insulated,  retain  more 
than  my  natural  share. 

/.  And  what  is  given  by  you  to  me  is  likewise  in- 
stantaneously supplied  by  the  earth  ? 

T.  It  is.  Let  us  make  another  experiment  to 
shew  that  the  electric  fluid  is  taken  from  the  earth. 
Here  are  some  little  balls  made  of 
the  pith  of  elder  :  they  are  put  on 
thread,  and,  being  very  light,  are 
well  adapted  to  our  purpose. 

While  the  chain  is  on  the  cushion, 
and  I  work  the  machine,  do  you 
bring  the  balls  near  the  conductor 
by  holding  the  thread  at  D.  ^..^ 
/ .  They  are  attracted  by  it,  and  ,  ^ 

now  the  two  balls  repel  each  other,       Fig.  3. 
as  in  the  figure  x. 

T.  I  ought  to  have  told  you,  that  the  upper  part  d 
of  the  line  is  silk,  by  which  means  you  know  the 
balls  are  insulated,  as  silk  is  a  non-conductor.   I  take 
the  chain  off  from  the  cushion,  and  put  it  on  the  con- 
ductor, so  as  to  hang  on  the  ground,  while  I  turn  the 
machine.    Will  the  balls  be  affected  now,  if  you 
hold  them  to  the  conductor? 
/.  No,  they  are  not. 
T.  Take  them  to  the  cushion. 
C.  They  are  attracted  and  repelled  now  by  being 
brought  near  the  cushion,  as  they  were  before  by 
being  carried  to  the  conductor. 

T.  Yes,  and  you  may  now  take  sparks  from  the 
cushion  as  you  did  just  now  from  the  conductor  ;  in 


446 


ELECTRICITY. 


both  cases  it  must  be  evident  that  the  electric  fluid  is 
brought  from  the  earth. 

Some  machines  are  furnished  with  two  conductors, 
one  of  which  is  connected  with  the  cushion,  the  other 
such  as  we  have  described.  Turn  the  cylinder,  and 
both  conductors  will  be  electrified ;  but  any  body 
which  is  brought  within  the  influence  of  these,  will  be 
attracted  by  one  of  the  conductors,  and  repelled  by 
the  other:  and,  if  a  chain  or  wire  be  made  to  con- 
nect the  two  together,  neither  will  exhibit  any  electric 
appearances  :  they  seem,  therefore,  to  be  in  opposite 
states ;  accordingly,  electricians  say,  that  the  conductor 
connected  with  the  cushion  is  negatively  electrified, 
and  the  other  is  positively  electrified. 

Machines  of  this  kind  have  been  used  for  medicinal 
purposes,  but  not  hitherto  with  much  success.  They 
have  been  principally  had  recourse  to  in  palsy,  con- 
tractions of  the  limbs,  rheumatism,  St.  Vitus's  dance, 
and  some  cases  of  deafness  and  imoaired  vision,  to 
remove  local  pain 


CONVERSATION  V. 

OF  ELECTRICAL  ATTRACTION  AND  REPULSION. 

J.  What  is  this  large  roll  of  sealing-wax  for  1 
T.  As  I  mean  to  explain,  this  morning,  the  prin- 
ciples of  electrical  attraction  and  repulsion,  I  have, 
besides  the  electrical  machine,  brought  out  for  use  a 
roll  of  sealing-wax,  which  is  about  fifteen  inches  long, 
and  an  inch  and  a  quarter  in  diameter,  and  the  long 
glass  tube. 

C.  Are  they  not  both  electrics,  and  capable  of 
being  excited? 

r.  They  are ;  but  the  electricity  produced  by  ex- 
citing them  has  different  or  contrary  properties. 

J,  Are  there  two  kinds  of  electricities  then  ? 

T,  We  will  shew  you  an  experiment  before  we  at- 
tempt to  give  any  theory.^ — I  will  excite  the  glass  tube, 


OF  TWO  ELECTRICITIES.  447 
and  Charles  shall  excite  the  wax ;  now  do  you  bring 
the  pith-balls,  which  are  suspended  on  silk  (Fig.  3.) 
to  the  tube :  they  are  suddenly  drawn  to  it,  and  now 
they  are  repelled  from  one  another,  and  likewise  from 
the  tube,  for  you  cannot  easily  make  them  touch  it 
again  : — but  take  them  to  the  excited  wax. 

/ .  The  wax  attracts  them  very  powerfully  :  now 
they  fall  together  again,  and  appear  in  the  same  state 
as  they  were  in  before  they  were  brought  to  the  excited 
tube. 

Repeat  the  experiment  again  and  again,  because 
on  this  two  different  theories  have  been  formed  :  one 
of  which  is,  that  there  are  two  electricities,  called  by 
some  philosophers  the  vitreous  or  positive  electricity, 
and  the  resinous  or  negative  electricity. 

C.  Why  are  they  called  vitreous  and  resinous  7 
T,  The  word  vitreous  is  Latin,  and  signifies  any 
glassy  substance ;  and  the  word  resinous,  used  to  denote 
that  the  electricity  produced  by  resins,  wax,  &c. 
possesses  different  qualities  from  that  produced  by 
glass. 

/.  Is  it  not  natural  to  suppose  that  there  are  two 
electricities,  since  the  excited  wax  attracts  the  very 
same  bodies  that  the  excited  glass  repels  ? 

T,  It  may  be  easily  explained,  by  supposing  that 
every  body,  in  its  natural  state,  possesses  a  certain 
quantity  of  the  electric  fluid,  and  if  a  part  of  it  be 
taken  away,  it  endeavours  to  get  it  from  other  bodies  ; 
or  if  more  be  thrown  upon  it  than  its  natural  quantity, 
it  yields  it  readily  to  other  bodies  that  come  within  its 
influence. 

C.  I  do  not  understand  this. 

T.  If  I  excite  this  glass  tube,  the  electricity  which 
it  exhibits  is  supposed  to  come  from  my  hand ;  but  if 
I  excite  the  roll  of  wax  in  the  same  way,  the  effect  is, 
according  to  this  theory,  that  a  part  of  the  electric 
fluid  naturally  belonging  to  the  wax  passes  from  it 
through  my  hand  to  the  earth  :  and  the  wax  being 
surrounded  by  the  air,  which,  in  its  dry  state,  is  a 


448 


ELECTRICITY. 


non-conductor,  remains  exhausted,  and  is  ready  to 
take  sparks  from  any  body  that  may  be  presented 
to  it. 

J.  Can  you  distinguish  that  the  sparks  come  from 
the  glass  to  the  hand ;  and,  on  the  contrary,  from  the 
hand  to  the  wax  1 

T.  No :  the  velocity  with  which  light,  and  of  course 
the  electric  spark,  moves,  renders  it  impossible  to  say 
what  course  it  takes  ;  but  I  shall  shew  you  other  ex- 
periments which  seem  to  j  ustify  this  theory :  and  as 
nature  always  works  by  the  simplest  means,  it  seems 
more  consistent  with  her  usual  operations,  that  there 
should  be  one  fluid  rather  than  two,  provided  that 
known  facts  can  be  equally  well  accounted  for  by 
one  as  by  two. 

C.  Can  you  account  for  all  the  leading  facts  by 
either  theory? 

T.  Yes,  we  can.  You  saw  when  the  pith-balls 
were  electrified,  they  repelled  one  another.  It  is 
a  general  principle  in  electricity,  that  two  bodies 
having  more  than  their  natural  share  of  the  electric 
fluid,  will  repel  one  another.  But  if  one  have  more, 
and  the  other  less,  than  its  share,  they  will  attract  one 
another. 

J.  How  is  this  shewn? 

T.  I  vv'ill  hold  this  ball,  which  is  insulated,  by  a 
silk  thread,  to  the  conductor,  and  do  you,  Charles,  do 
the  same  with  the  other.  Let  us  now  bring  them 
together. 

C.  No,  we  cannot :  they  fly  from  one  another. 

T,  I  will  hold  mine  to  the  insulated  cushion,  and 
you  shall  hold  yours  to  the  conductor,  while  the 
machine  is  turned :  now  I  suspect  they  will  attract 
one  another, 

J.  They  do  indeed. 

C.  The  reason  is  this  ;  that  the  cushion,  and  what- 
ever is  in  contact  with  it,  parts  with  a  portion  of  its 
electricity ;  but  the  conductor  and  the  adjoining 
bodies  have  more  than  their  share  j  therefore,  the 


ATTRACTION  AND  REPULSION.  449 
ball  applied  to  the  cushion,  being  negatively  electri- 
fied, will  attract  the  one  connected  with  the  con- 
ductor, which  is  positively  electrified. 

T.  Here  is  a  tuft  of  feathers,  which  I  stick  in  a 
small  hole  in  the  conductor :  now  see  what  happens 
when  I  turn  the  cylinder. 

/ .  They  all  endeavour  to  avoid  each  other,  and 
stand  erect  in  a  beautiful  manner.  Let  me  take  a 
spark  from  the  conductor ;  now  they  fall  down  in  a 
moment. 

T,  When  I  turned  the  wheel  they  all  had  more 
than  their  share  of  the  electric  fluid,  and  therefore 
they  repelled  one  another ;  but  the  moment  the  elec- 
tricity was  taken  away,  they  fell  into  their  natural 
position.  A  large  plume  of  feathers,  when  electrified, 
grows  beautifully  turgid,  expanding  its  fibres  in  all 
directions,  and  they  collapse  when  the  electricity  is 
taken  ofF.  ^ 

J .  Could  you  make  the  hairs  of  my  head  repel  one 
another  ? 

T.  Yes,  that  I  can.  Stand  on  the  glass-legged 
stool,  and  hold  the  chain  that  hangs  on  the  conductor, 
in  your  hand,  while  I  turn  the  machine. 

C.  Now  your  hairs  stand  all  on  end. 

J.  And  I  feel  something  like  cobwebs  over  my 
face. 

T.  There  are,  however,  no  cobwebs,  but  that  is  a 
sensation  which  a  person  always  experiences  if  he  is 
highly  electrified.  Hold  the  pith-ball,  Charles,  near 
your  brother's  face. 

/.  It  is  attracted  in  the  same  manner  as  it  was 
before  with  the  conductor. 

r.  Hence  you  may  lay  it  down  as  a  general  rule, 
that  all  light  substances  coming  within  the  influence 
[  of  an  electrified  body  are  attracted  by  it,  whether  it 
!  is  electrified  positively  or  negatively. 

C.  Because  they  are  attracted  by  the  positive  elec- 
tricity to  receive  some  of  the  superabundant  quan- 
tity ;  and  by  the  negative  to  give  away  some  that  they 
possess. 


450 


ELECTRICITY. 


T,  J ust  so :  and  when  they  have  received  as  much 
as  they  can  contain,  they  are  repelled  by  the  electri- 
fied body.  The  same  thing  may  be  shewn  in  various 
ways.  Having  excited  this  glass  tube,  either  by 
drawing  it  several  times  through  my  hand,  or  by 
means  of  a  piece  of  flannel,  I  will  bring  it  near  this 
small  feather.  See  how  quickly  it  jumps  to  the 
glass. 

J.  It  does,  and  sticks  to  it. 

T.  You  will  observe,  that  after  a  minute  or  two  it 
will  have  taken  as  much  electricity  from  the  tube  as 
it  can  hold,  when  it  will  suddenly  be  repelled,  and  ^ 
jump  to  the  nearest  conductor;  upon  which  it  will  , 
discharge  the  superabundant  electricity  that  it  has 
acquired. 

J.  I  see  it  is  now  going  to  the  ground,  that  being 
the  nearest  conductor. 

T,  I  will  prevent  it  by  holding  the  electrified  tube 
between  it  and  the  floor.    You  see  how  unwilling 
it  is  to  come  again  in  contact  with  the  tube :  by  pur-  ' 
suing,  I  can  drive  it  where  I  please  without  touch-  . 
ing  it. 

C,  That  is,  because  the  glass  and  the  feather  are  ' 
both  loaded  with  the  same  electricity?  t 

T.  Let  the  feather  touch  the  ground,  or  any  other  t 
conductor,  and  you  will  see  that  it  will  jump  to  the  I 
tube  as  fast  as  it  did  before.  - 

I  will  suspend  this  brass  plate,  which  is  about  five  / 
inches  in  diameter,  to  the  conductor,  and  at  the  dis-  ^ 
tance  of  three  or  four  inches  below  I  will  place  some  ^ 
small  feathers,  or  bits  of  paper  cut  into  the  figures  of 
men  and  women.    They  lay  very  quiet  at  present ; 
observe  their  motions  as  soon  as  I  turn  the  wheel. 

J.  They  exhibit  a  pretty  country  dance  :  they 
jump  up  to  the  top  plate,  and  then  down  again. 

T.  The  same  principle  is  evident  in  all  these  ex-i 
periments.  The  upper  plate  has  more  than  its  owuj 
share  of  the  electric  fluid,  which  attracts  the  littlel 
figures  ;  as  soon  as  they  have  received  a  portion  of  itJ 
they  go  down  to  give  it  to  the  lower  plate;  and  so  *" 


ATTRACTION  AND  REPULSION.  451 
will  continue  till  the  upper  plate  is  discharged  of  its 
superabundant  quantity. 

I  will  take  away  the  plates,  and  hang  a  chain  on 
the  conductor,  the  end  of  which  shall  lie  in  several 
folds  in  a  glass  tumbler;  if  I  turn  the  machine,  the 
electric  fluid  will  run  through  the  chain,  and  will 
electrify  the  inside  of  the  glass.  This  done,  I  turn  it 
quickly  over  eight  or  ten  small  pith-balls,  which  lie 
on  the  table. 

C.  That  is  a  very  amusing  sight;  how  they  jump 
about!  They  serve  also  to  fetch  the  electricity  from 
the  glass  and  carry  it  to  the  table. 

T,  If,  instead  of  the  lower  metal  plate,  I  hold  in 
my  hand  a  pane  of  dry  and  very  clean  glass,  by  the 
corner,  the  paper  figures,  or  pith-balls,  will  not  move, 
because  glass  being  a  non-conducting  substance,  it 
has  no  power  of  carrying  away  the  superabundant 
electricity  from  the  plate  suspended  from  the  con- 
ductor. But  if  I  hold  the  glass  flat  in  my  hand,  the 
figures  will  be  attracted  and  repelled,  which  shews 
that  the  electric  fluid  will  pass  through  thin  glass. 

Take  now  the  following  results,  and  commit  them 
to  your  memory : — • 

(1.)  If  two  insulated  pith-balls  be  brought  near 
the  conductor,  they  will  repel  each  other. 

(2.)  If  an  insulated  conductor  be  connected  with 
the  cushion,  and  two  insulated  pith-balls  be  electrified 
by  it,  they  will  repel  each  other. 

(3.)  If  one  insulated  ball  be  electrified  by  the 
prime  conductor,  and  another  by  the  conductor  con- 
nected with  the  cushion,  they  will  attract  each  other. 

(4.)  If  one  ball  be  electrified  by  glass,  and  another 
by  wax,  they  will  attract  each  other. 

(5.)  If  one  ball  be  electrified  by  a  smooth,  and 
another  by  a  rough,  excited  glass  tube,  they  will 
attract  one  another. 


452 


ELECTRICITY. 


CONVERSATION  VI. 

OF  ELECTRICAL  ATTRACTION  AND  REPULSION. 

T,  I  will  shew  you  another  instance  or  two  of 
the  effects  of  electrical  attraction  and  repulsion. 

This  apparatus  consists 
of  three  bells  suspended 
from  a  brass  wire,  the  two 
outer  ones  by  small  brass 
chains;  the  middle  bell, 
and  the  two  clappers  .t  t, 
are  suspended  on  silk. 
From  the  middle  bell  there 
is  a  chain  n,  which  goes 
to  the  table,  or  any  other 
conducting  substance.  The  j'io-.  4. 

bells  are  now  to  be  hung  ° 
by  c  on  the  conductor,  and  the  electrical  machine 
to  be  put  in  motion. 

/.  The  clappers  go  from  bell  to  bell,  and  make 
very  pretty  music  :  how  do  you  explain  this  ? 

T.  The  electric  fluid  runs  down  the  chains  a  and  b  ; 
to  the  bells  a  b,  these,  having  more  than  their  natural  I 
quantity,  attract  the  clappers  x  x,  which  take  a  por-  I 
tion  from  a  and  b,  and  carry  it  to  the  centre  bell  n,  i 
and  this,  by  means  of  the  chain,  conveys  it  to  the  i 
earth. 

C.  Would  not  the  same  effect  be  produced  if  the 
clappers  were  not  suspended  on  silk  ? 

T.  Certainly  not :  nor  will  it  be  produced  if  the 
chain  be  taken  away  from  the  bell  n,  because  then 
there  is  no  way  left  to  carry  off  the  electric  fluid  to^ 
the  earth,  ■ 

Another  amusing  experiment  is  thus  shewn  :  Let  I 
there  be  two  wires  placed  exactly  one  above  another,  ■ 
and  parallel  ;  the  upper  one  must  be  suspended  fromB 
the  conductor,  the  other  is  to  communicate  with  thai 
table  :  a  light  ima^^e  placed  between  these  will,  wheuB 


ELECTROMETERS.  453 
the  conductor  is  electrified,  appear  like  a  rope  dancer. 
—  This  piece  of  leaf-brass  is  called  the  electric  Jish ; 
one  end  is  a  sort  of  obtuse  angle,  the  other  is  acute  • 
if  the  large  end  be  presented  towards  an  electrified 
conductor,  it  will  fix  to  it,  and,  from  its  wavering 
motion,  it  will  appear  to  be  animated. 

This  property  of  attraction  and  repulsion  has  led  to 
many  inventions  of  instruments  called  electrometers. 

J.  Is  not  an  electrometer  a  machine  to  measure 
the  strength  of  the  electricity  ? 
^  T ,  Yes ;  and  this  is  one  of  the  most 
simple,  and  it  depends  entirely  upon 
the  repulsion  which  takes  place  be- 
tween  two  bodies  in  a  state  of  electrifi- 
cation.   It  consists  of  a  lio-ht  rod  and 


a  pith-ball,  hanging  parallel  to  the 

stem,  but  turning  on  the  centre  of  a 

semi- circle,  so  as  to  keep  close  to  its 

graduated  limb.    This  is  to  be  placed 

in  a  hole  on  the  conductor  l,  (Fig.  2.) 

and  according  as  the   conductor  is 

more  or  less  electrified,  the  ball  will  fly       Fig.  5. 

farther  from  the  stem, 

C.  If  the  circular  part  be  marked  with  degrees, 

you  may  ascertain,  I  suppose,  pretty  accurately,  the 
strength  of  any  given  charge. 

T,  Yes,  you  may ;  but  you  see  how  fast  the  air 
carries  away  the  electricity;  it  scarcely  remains  a 
single  moment  in  the  place  to  which  it  was  repelled. 
—Two  pith-balls  may  be  suspended  parallel  to  one 
another,  on  silken  threads,  and  applied  to  any  part  of 
an  electrical  machine,  and  they  will,  by  their  repul- 
sion, serve  for  an  electrometer,  for  they  will  repel  one 
fiSr^^^  machine  acts  more  power- 

Has  this  any  advantage  over  the  other  ? 
T,  It  serves  to  shew  whether  the  electricity  be 
I  negative  or  positive  ;  for  if  it  be  positive,  by  applying 
an  excited  stick  of  sealing-wax,  the  threads  will  fall 
jtogether  again ;  but  if  it  be  negative,  excited  sealing- 


454 


ELECTRICITY. 


wax,  or  resin,  or  sulphur,  or  even  a  rod  of  glass  (the 
polish  of  which  is  taken  off),  will  make  them  recede 
farther. 

We  have  now,  perhaps,  said  enough  respecting 
electrical  attraction  and  repulsion,  at  least  for  the 
present ;  I  wish  you,  however,  to  commit  the  follow- 
ing results  to  your  memory: — 

1.  Bodies  that  are  electrified  positively  repel  each 
other. 

2.  Bodies  that  are  electrified  negatively  repel  each 
other. 

C.  Do  you  mean,  that  if  two  bodies  have  either 
more  or  less  of  the  electric  fluid  than  their  natural 
share,  they  will  repel  each  other,  if  brought  suf- 
ficiently near  1 

T.  That  is  exactly  what  I  mean. 

3.  Bodies  electrified  by  contrary  powers,  that  is, 
two  bodies,  one  having  more,  and  the  other  less,  than 
its  natural  share,  attract  each  other  very  strongly. 

4.  Bodies  that  are  electrified  attract  light  sub- 
stances which  are  not  electrified. 

These  are  facts  which,  I  trust,  have  been  made 
evident  to  your  senses.  To-morrow  we  will  describe 
what  is  usually  called  the  Leyden  phial. 


CONVEKSATION  VII. 

OF  THE  LEYDEN  PHIAL,   OR  JAR. 

T.  I  will  take  away  the  wires  and  the  ball  from  the 
conductor,  and  then  remove  the  conductor  an  inch  or 
two  farther  from  the  cylinder.    If  the  machine  act 
strongly,  bring  an  insulated  pith-ball,  that  is,  yo 
know,  one  hanging  on  silk,  to  the  end  of  the  conduc 
tor  nearest  to  the  glass  cylinder. 

C.  It  is  immediately  attracted. 

T.  Carry  it  to  the  other  end  of  the  conductor,  an 
see  what  happens. 

C.  It  is  attracted  again,  but  I  thought  it  woul 
have  been  repelled. 


LEYDEN  PHIAL.  455 
T,  Then  as  the  ball  was  electrified  before,  and  is 

still  attracted,  you  are  sure  that  the  electricity  of  the 

two  ends  of  the  conductor  are  of  different  names  ; 

that  is,  one  is  jflus,  and  the  other  minus.  ' 
J.  Which  is  the  positive,  and  which  the  nerative 

end?  ^ 

T.  That  end  of  the  conductor  which  is  nearest  to 
the  cylinder  becomes  possessed  of  an  electricity  dif- 
ferent from  that  of  the  cylinder  itself. 

/.  Do  you  mean,  that  if  the  cylinder  is  positively 
electrified,  the  end  of  the  conductor  next  to  it  is 
electrified  negatively  1 

T,  I  do  :  and  this  you  may  see  by  holding  an  in- 
sulated pith-ball  between  them. 

C,  Yes,  it  is  now  very  evident,  for  the  ball  fetches 
and  carries  as  we  have  seen  it  before. 

r.  What  you  have  seen  with  regard  to  the  conduc- 
tor, is  equally  true  with  respect  to  non-conducting 
bodies  :^  here  is  a  common  glass  tumbler  ;  if  I  throw 
withinside  it  a  greater  portion  of  electricity  than  its 
natural  share,  and  hold  it  in  my  hand,  or  place  it  on 
any  conducting  substance,  as  the  table,  a  part  of  the 
electric  fluid,  that  naturally  belongs  to  the  outside, 
I  will  make  its  escape  through  my  body. 

C.  Let  me  try  this. 
I    T,  But  you  must  be  careful  that  you  do  not  break 
!the  glass. 

i  C,  I  will  hang  the  chain  on  the  conductor,  and  let 
'the  other  end  lie  on  the  bottom  of  the  glass,  and 
iJames  will  turn  the  machine. 

T.  You  must  take  care  that  the  chain  does  not 
touch  the  edge  of  the  glass,  because  then  the  electric 
fluid  will,  by  that  means,  run  from  one  side  of  it  to 
the  other,  and  spoil  the  experiment. 
I  /.  If  I  have  turned  the  machine  enough,  take  the 
chain  out,  and  try  the  two  sides  with  the  insulated 
Ipith-ball. 

I   C.  What  is  this?  something  has  pierced  throuo-h 
|ny  arms  and  shoulders.  ° 


456  ELECTRICITY. 

T,  That  is  a  trifling  electrical  shock,  which  you 
might  have  avoided,  if  you  had  vi^aited  for  my  di- 
rections. 

C.  Indeed  it  ^zs  not  trifling:  I  feel  it  now. 

T,  This  leads  us  to  the  Leyden  phial  :  so  called, 
because  the  discovery  was  first  made  at  Leyden,  in 
Holland,  and  by  means  of  a  phial  or  small  bottle. 

J.  Was  it  found  out  in  the  same  manner  as  Charles 
has  just  discovered  it] 

T.  Nearly  so  :  Mr.  Cuneus,  a  Dutch  philosopher, 
was  holding  a  glass  phial  in  his  hand,  about  half 
filled  with  water,  but  the  sides  above  the  water  and 
the  outside  were  quite  dry ;  a  wire  also  hung  from  the 
conductor  of  an  electrical  machine  into  the  water. 

J.  Did  that  answer  to  the  chain  ? 

T,  Just  so  :  and,  like  Charles,  he  was  going  to 
disengage  the  wire  with  one  hand,  as  he  held  the 
bottle  in  the  other,  and  was  surprised  and  alarmed  by 
a  sudden  shock  in  his  arms,  and  through  his  breast, 
which  he  had  not  the  least  expected. 

C.  I  do  not  think  there  was  any  thing  to  be 
alarmed  at. 

T.  The  shock  which  he  felt  was,  probably,  some- 
thing severer  than  that  which  you  have  just  ex- 
perienced :  but  the  terror  was  evidently  increased  by 
its  coming  so  completely  unexpected. 

When  M.  Muschenbroeck  first  felt  the  shock, 
which  was  by  means  of  a  thin  glass  bowl,  and  very 
slight,  he  wrote  to  Reaumur,  that  he  felt  himself 
struck  in  his  arms,  shoulders,  and  breast,  so  that  he 
lost  his  breath,  and  was  two  whole  days  before  he 
recovered  from  the  effects  of  the  blow.  •  fl 

C.  Perhaps  he  meant  the  fright  ]  \ 

r.  Terror  seems  to  have  been  the  effect  of  the 
shock :  for  he  adds,  '*  I  would  not  take  a  second 
shock  for  the  whole  kingdom  of  France."  i 

Mr.  Ninkler,  an  experimental  philosopher  a| 
I.eipsic,  describes  the  shock  as  having  given  hidj 
convulsions,  a  heaviness  in  his  head,  such  as  he  shoull 


OF  THE  LEYDEN  JAR.  4.7 
feel  if  a  large  stone  were  on  it,  and  he  had  reason  to 
dread  a  fever  to  preveat  which  he  put  himself  0",  ^ 
course  of  coolmg  medicines.  "  Twice  "  saTs  he 
gave  me  a  bleeding  at  the  nose  to  wh'i.K T  '  ' 
mclined;  and  my  life,  wLrc^-foslIy'tpasL"  hel 
tears,  received  the  shock  twice  and  fmmH  ^ 

ThfcoS  ot  fe""':!  --W^^  atthX 

--^er  shoe. 

J;  Ts  this  called  the  Leyden  phial  ? 
Itis.  1  hey  are  now  made  in 
this  manner,  a  is  a  glass  jar,  both 
inside  and  out  being  covered  with 
tin-foil  about  three  parts  of  the 
way  up,  as  far  as  x. 

C,  Does  the  outside  coverino- 
answer  to  the  hand,  and  the  inside  ^^^..nm 
covering  to  the  water?  j,-^  g 

,  They  do  :  the  piece  of  wood  * 
2  IS  placed  on  the  top,  merely  to  support  the  bras, 
wire  and  knob  to  the  bottom  of  whl  hanl  1 
Cham  that  rests  on  the  bottom  of  the  ja^  I  will  fow 
set  the  jar  m  such  a  situation  that  it  shall  be  withi^ 

i::zZ7/''''''    conductor  whiie'Ti';;: 

the^knob'  ''P^'^^^  ^'^^^        ^^"ductor  to 

C.  The  brass  wire  touches  the  inside  :  if  I,  there- 
X 


453  ELECTRICITY. 

fore,  with  one  hand  touch  the  knob,  and  with  the 

other  the  outside  covering,  will  it  be  sufficient '? 

T     It  will :  but  I  had  rather  you  would 
not,  because  the  shock  will  be  more  power- 
ful  than  I  should  wish  either  myself  or  you  ji 
to  experience.    Here  is  a  brass  wire  with  fj 
two  little  balls  or  knobs  b  s  to  it.    I  will 
brino-  one  of  them,  as  s,  to  the  outside,  and    l4g.  /. 
the  other,  6,  to  the  ball  v  on  the  wire.      ,     .     .  , 
J.  What  a  brilliant  spark  and  what  a  loud  noise  ! 
T  The  electric  fluid  that  occasions  the  light  and 
tlie  noise,  ran  from  the  inside  of  the  jar  through  the 
wire  to  s,  and  spread  itself  over  the  outside. 

C.  Would  it  have  gone  through  my  arms,  it  i  had 
put  one  hand  to  the  outside,  and  touched  the  wire 
communicating  with  the  inside  with  the  other  ? 

r.  It  would,  and  you  may  believe  the         ;  , 
shock  would  have  been  in  proportion  to 
the  quantity  of  fluid  collected.    Ihe  in- 
strument I  used  may  be  called  a  discharg-      ^  ^ 
incr  rod :  but  here  is  a  more  convenient     ,  ^ 
one :  the  handle  a  is  solid  glass,  fastened 
into  a  brass  socket,  and  the  brass  work  is 
the  same  as  fig.  7,  only  by  turnmg  on  a    Fig.  8, 
joint  the  arms  may  be  opened  to  any  extent. 
J,  Why  is  the  handle  glass  1 
T.  Because  glass  being  a  non-conductor  the  elec. 
trie  fluid  passes  through  the  brass  work,  withou 
affectinc.  the  hand;  whereas  with  the  other  a  small 
sensau^^^^  was  percJived  while  I  discharged  the  jar. 
C  Would  the  jar  never  discharge  itselt  { 
T  Yes  :  by  exposure  to  the  air  for  some  time 
the  charge  of  the  jar  will  be  silently  and  graduall 
dissipated,  for  the  superabundant  electric  fluid  of  th 
ins  rlil  escape,  by  means  of  the  air,  to  the  outsid 
of  the  iar.-But  dectricians  make  it  a  rule  never 
kave  a  jar  in  its  charged  state,  lest  any  person  comm 
nto  the  room  unawares  should  happen  to  touch 
and  thereby  receive  a  shock  which  might  be  attend 
with  serious  consequences. 


ELECTRICAL  BATTERY. 


459 


CONVERSATION  VIII. 

OF  THE   LEYDEN   JAR— LANe's   DISCHARGING  FLEC^ 
TROMETER,   AND  THE   ELECTRICAL  BATTERY. 

C  In  discharging  the  jar  yesterday,  I  observed 
that  when  one  of  the  discharging-  rods  touched  the 
outside  of  the  jar,  the  flash  and  report  took  place 
betore  the  other  end  came  in  contact  with  the  brass 
wire  that  communicates  with  the  inside  coating. 

les,  It  acts  in  the  same  manner  as  when  vou 
take  a  spark  from  the  conductor;  you  do  not/for 
that  purpose,  bring  your  knuckle  close  to  the  tin 

J.  .Sometimes,  when  the  machine  acts  very  power- 
l^^^l^J''''        get  the  spark  at  the  distance  of  several 

nr  j'.  ^/  ^^^^  principle,  the  higher  an  electrical 
01  Leyden  jar  is  charged,  the  more  easily,  or  at  a 
greater  distance,  is  it  discharged. 

-UA^'T  experiments  it  does  not  seem  that  it 
will  discharge  at  so  great  a  distance  as  that  in  which 
a  spark  may  be  taken  from  the  conductor. 

i.  Very  frequently  a  jar  will  discharge  itself,  after 
It  has  accumulated  as  much  of  the  electrical  fluid 
asit  cancontam:  that  is,  the  fluid  which  is  thrown 
on  the  mside  coating  will  make  its  way  over  the 
foating  ^  non-conductor,  on  to   the  outside 

J.  In  a  Leyden  jar,  after  the  first  discharge,  vou 
always  I  perceive,  take  another  and  smaller  on^. 
rin.;./.!  ;  ''''  ^^^^^  ^  Pe^-fect  con. 

first  ^;nrl'%r^  ' ^^^^^^^  pass  at 

ca  L  r     !/  ^^^^  remains  is 

called  the  residuum,  and  this,  in  a  large  jar.  would 
give  you  a  considerable  shock;  therefore,  I  advise 
you  always,  in  discharging  an  electrical  jar,  to  take 
away  the  residuum  before  you  venture  to  remove  the 
apparatus.    I  will  now  describe   an  electrometer 


460 


ELECTRICITY. 


which  depends,  for  its  action,  on  the  principles  we 
have  been  describing. 

C.  Do  you  mean  upon  the  jar's  discharging  before 
the  outside  and  inside  coating  are  actually  brought 
into  contact? 

J".  I  do  :  the  arm  d  is  made 
of  glass,  and  proceeds  from  a 
socket  on  the  wire  of  the  elec- 
trical jar  F.  To  the  top  of  the 
glass  arm  is  cemented  another 
brass  socket  e,  through  which 
a  wire,  with  balls  b  and  c  at 
each  end,  will  slide  backwards 
and  forwards. 

J.  So  that  it  may  be  brought  Fig.  9. 

to  any  distance  from  the  ball  a, 
which  is  on  the  wire,  connected  with  the  inside  of  the 
jar? 

T.  Just  so.  When thej arris  set  either  in  contact 
with,  or  very  near,  the  conductor,  as  it  is  represented  in 
the  figure,  and  the  ball  b  is  set  at  the  distance  of  the 
eighth  of  an  inch  from  the  ball  a,  let  a  wire  c  k  be 
fixed  between  the  ball  c  and  the  outside  coating  of 
the  jar.  Then  as  soon  as  the  machine  is  worked,  the 
jar  cannot  be  charged  beyond  a  certain  point,  for  when 
the  charge  is  strong  enough  to  pass  from  a  to  the 
ball  E,  the  discharge  will  take  place,  and  the  electric 
fluid  collected  in  the  inside  will  pass  through  the  wire 
c  K  to  the  outside  coating. 

C.  If  you  remove  the  balls  to  a  greater  distance 
from  one  another,  will  a  stronger  charge  be  required 
before  the  fluid  can  pass  from  the  inside  of  the  jar  to 
the  ball  b  of  the  electrometer  1 

T.  Certainly :  and  therefore  the  discharge  will  be 
much  stronger.  This  machine  is  called  Lane's  Dis- 
charging Electrometer,  from  the  name  of  the  person 
who  invented  it.  It  is  very  useful  in  applying  the 
electric  shock  to  medical  purposes,  as  we  shall  see] 
hereafter.  j 


ELECTRICAL  BATTERY. 


461 


Fig.  10. 

This  box  contains  nine  jars  or  Leyden  phials ;  the 
wires,  which  proceed  from  the  inside  of  each  three  of 
these  jars,  are  screwed  or  fastened  to  a  common 
horizontal  wire  e,  which  is  knobbed  at  each  extremity, 
and  by  means  of  the  wires  f  f  the  inside  coatings  of 
three  or  six,  or  the  whole  nine,  may  be  connected. 

J.  Is  it  a  common  box  in  which  the  jara  are 
placed  1 

T,  The  inside  of  the  box  is  lined  with  tin-foil  ; 
sometimes  very  thin  tin-plates  are  used,  for  the  pur- 
pose of  connecting,  more  effectually,  the  outside  coat- 
ings of  all  the  jars. 

C.  What  is  the  hook  on  one  of  the  sides  of  the 
box  for  ? 

T,  To  this  hook  is  fastened  a  strong  wire,  which 
communicates  with  the  inside  lining  of  the  box,  and, 
of  course,  with  the  outside  coating  of  the  jars.  And, 
as  you  see,  to  the  hook  a  wire  is  also  fastened,  which 
connects  it  with  one  branch  of  the  discharging  rod. 

J,  Is  there  any  particular  art  to  be  used  in  charg- 
ing a  battery  ? 

T.  No:  the  best  way  is,  to  bring  a  chain,  or  piece 
of  wire,  from  the  conductor  to  one  of  the  balls  on 
the  rods  that  rest  upon  the  jars :  and  then  set  the 
machine  to  work.  The  electric  fluid  passes  from 
the  conductor  to  the  inside  of  all  the  jars,  till  it  is 
charged  sufficiently  high  for  the  purpose.  Great 
caution,  however,  must  be  used  when  you  come  to 


462 


ELECrRIClTY. 


make  experiments  with  a  battery,  for  fear  of  an 
accident,  either  to  yourself,  or  to  spectators. 

C.  Would  a  shock  from  this  be  attended  with  any 
bad  consequences  1 

T.  Yes  :  very  serious  accidents  may  happen  from 
the  electricity  accumulated  in  a  large  battery,  and 
even  with  a  battery  such  as  is  represented  in  the 
figure,  which  is  one  of  the  smallest  made :  a  shock 
may  be  given,  which,  if  passed  through  the  head  or 
other  vital  parts  of  the  body,  may  be  attended  with 
very  mischievous  effects. 

/.  How  do  you  know  when  the  battery  is  properly 
charged  ? 

T,  The  quadrant  electrometer  (Fig.  5.)  is  the  best 
guide,  and  this  may  be  fixed  either  on  the  conductor, 
or  upon  one  of  the  rods  of  the  battery.  But  if  it  is 
fixed  on  the  battery,  the  stem  of  it  should  be  of  a 
good  length,  not  less  than  12  or  15  inches. 

C.  How  high  will  the  index  stand  when  the  battery 
is  charged  1 

T.  It  will  seldom  rise  so  high  as  90^,  because  a 
machine,  under  the  most  favourable  circumstances, 
cannot  charge  a  battery  so  high,  in  proportion,  as  a 
single  jar.  You  may  reckon  that  a  battery  is  well 
charged  when  the  index  rises  as  high  as  60^,  or 
between  that  and  70''. 

J.  Is  there  no  danger  of  breaking  the  jars  when 
the  battery  is  very  highly  charged  1 

T,  Yes,  there  is  j  and  if  one  jar  be  cracked,  it  is 
impossible  to  charge  the  others,  till  the  broken  one  be 
removed :  to  prevent  accidents,  it  is  recommended 
not  to  discharge  a  battery  through  a  good  conductor, 
except  the  circuit  is  at  least  five  feet  long. 

C.  Do  you  mean  the  wire  should  be  so  long  ? 

T,  Yes,  if  you  pass  the  charge  through  that :  but 
you  may  carry  it  through  any  conductor.  j 

Before  a  battery  is  used,  the  uncoated  part  of  the! 
jars  must  be  made  perfectly  clean  and  dry,  for  the! 
smallest  particles  of  dust  will  carry  away  the  electricj 


EXPERIPJENTS.  463 
fluid.  And  after  an  explosion,  take  care  always  to 
connect  the  wire  on  the  hook  with  the  ball,  to  pre- 
vent any  residuum  from  remaining. 

/.  Have  not  small  animals  been  sometimes  killed 
by  an  electrical  battery  ? 

T.  Yes:  small  animals,  such  as  mice,  sparrows 
and  pigeons,  are  instantly  killed  by  a  shock  from 
thirty  square  inches  of  glass ;  and  even  if  a  man 
receive  a  charge  through  the  spine,  he  loses  his  power 
over  the  muscles  to  such  a  degree,  as  to  cause  him 
to  fall  prostrate  on  the  ground,  and  the  charge  may 
be  made  sufficiently  powerful  to  produce  immediate 
death. 

C.  Have  any  individuals  ever  been  killed  by  over- 
charging the  battery  ? 

T,  The  celebrated  Professor  Richmann  was  acci- 
dentally killed  by  a  stroke  of  lightning,  whilst  makino- 
experiments  on  the  clouds.  ° 


CONVERSATION  IX, 

EXPERIMENTS  MADE  WITH  THE  ELECTRICAL  BATTERY. 

r.  I  will  now  shew  you  some  experiments  with  this 
large  battery  :  to  perform  these  in  perfect  safety,  I 
must  beg  you  to  stand  a  good  distance  from  it:  this 
will  prevent  accidents. 

Ex.  1.  I  take  this  quire  of  writing-paper,  and  place 
It  against  the  hook  or  wire  that  comes  out  of  the  box  • 
and  when  the  battery  is  charged,  I  put  one  ball  of 
the  discharging  rod  to  a  knob  of  one  of  the  wires  r 
and  bring  the  other  knob  to  that  part  of  the  paper 
that  stands  against  the  wire  proceeding  from  the  box: 
you  see  what  a  hole  it  has  made  through  every  sheet 
of  the  paper.  Smell  the  paper  where  the  perfora- 
tion is. 

C.  It  smells  like  sulphur. 

T.  Or  more  like  phosphorus:  you  observe  in  this 
experiment,  that  the  electric  fluid  passed  from  the 


464  ELECTRICITY, 
inside  of  the  jars  through  the  conducting  rod  and  pa- 
per, to  the  outside. 

J.  Why  did  it  not  pass  through  the  paper  in  the 
same  manner  as  it  passed  the  brass  discharging  rod, 
in  which  it  made  no  hole? 

T.  Paper  is  a  non-conducting  substance,  but  brass 
is  a  conductor  :  through  the  latter  it  passes  without 
any  resistance,  and  in  its  endeavour  to  get  to  the  in- 
side of  the  box,  it  burst  the  paper  as  you  see  :  the 
same  thing  would  have  happened  had  there  been 
twice  or  thrice  as  much  paper.  The  electric  fluid  of 
a  single  jar  will  pierce  through  many  sheets  of 
paper. 

C.  Would  it  serve  any  other  non-conducting  sub- 
stance in  the  same  manner  ? 

T.  Yes  ;  it  will  even  break  a  thin  piece  of  glass,  or 
of  resin,  or  of  sealing-wax,  if  they  be  interposed  be- 
tween the  dischargmg  rod  and  the  outside  of  the 
coating  of  the  battery. 

Ex.  2.  Place  a  piece  of  loaf  sugar  in  the  situation 
m  which  the  quire  of  paper  was  just  now,  the  sugar 
will  be  broken,  and  in  the  dark  it  will  appear  beau- 
tifully illuminated,  and  remain  so  for  many  seconds  of 
time. 

Ex.  3.  Let  the  small  piece  of  wire,  proceeding 
from  the  hole  in  the  box,  be  laid  on  one  side  of  a 
plate,  containing  some  spirits  of  wine,  and  on  the 
opposite  side  of  the  plate  bring  one  of  the  knobs  of 
the  discharging  rod,  while  the  other  is  carried  to  the 
wires  connected  with  the  inside  of  the  jars. 

C.  I'hen  the  electric  fluid  will  have  a  passage 
through  the  spirit  1 

T.  It  will  set  it  on  fire  instantly. 

Ex.  4.  Take  two  slips  of  common  window-glass, 
about  four  inches  long,  and  one  inch  broad  ;  put  a 
slip  of  gold  leaf  between  the  glasses,  leaving  a  small 
part  of  it  out  at  each  end ;  then  tie  the  glasses  toge- 
ther, or  press  them  with  a  heavy  weight,  and  send  the 
charge  of  the  battery  through  it,  by  connecting  one 
end  of  the  glass  with  the  outside  of  the  jars,  and 


EXPERIMENTS. 


405 


bringing  the  discharging  rod  to  the  other  end,  and  to 
the  wires  of  the  inside  of  the  battery. 
/.  W  ill  it  break  the  glass  ? 

r.  It  probably  will;  but  whether  it  does  or  not, 
the  gold  leaf  will  be  forced  into  the  pores  of  the  glass, 
so  as  to  appear  like  glass  stained  with  gold,  which 
nothing  can  wash  away. 

Ex.  5.  If  the  gold  leaf  be  put  between  two  cards, 
and  a  strong  charge  passed  through  it,  it  will  be  com- 
pletely fused  or  melted,  the  marks  of  which  will  ap- 
pear on  the  card. 

This  instrument,  called  a  universal  discharger,  is 


Fig.  11. 


very  useful  for  passing  charges  through  many  sub- 
stances. B  B  are  glass  pillars  cemented  into  the  frame 
A.  To  each  of  the  pillars  is  cemented  a  brass  cap, 
and  a  double  joint  for  horizontal  and  vertical  mo- 
tions ;  on  the  top  of  each  joint  is  a  spring  tube,  which 
holds  the  sliding  wires  x,  x,  so  that  they  may  be 
set  at  various  distances  from  each  other,  and  turned 
in  any  direction  ;  the  extremities  of  the  wires  are 
pointed,  but  with  screws,  at  about  half  an  inch  from 
the  points,  to  receive  balls.  The  table  ed,  inlaid 
with  a  piece  of  ivory,  is  made  to  move  up  and  down 
in  a  socket,  and  a  screw  fastens  it  to  any  required 
height.  The  rings  c  c  are  very  convenient  for  fixing 
a  chain  or  wire  to  them,  which  proceeds  from  the 
conductor. 

C.  Do  you  lay  anything  on  the  ivory,  between  the 
balls,  when  you  want  to  send  the  charge  of  a  battery 
through  it  ? 

T.  Yes ;  and  by  drawing  out  the  wires,  the  balls 
X  2 


406  ELECTRICITY. 

may  be  separated  to  any  distance  less  than  the  length 
of  the  ivory.    The  little  figure  ri  repre- 
sents a  press,  which  may  be  substituted   Jf 

in  the  place  of  the  table  e  d  :  it  consists  (^IIjiZZ2; 
of  two  flat  pieces  of  mahogany,  which         I  I 
may  be  brought  together  by  screws.  Fig.  12. 

J.  Then,  instead  of  tying  the  slips  of 
glass  together  in  Ex.  4,  you  might  have  done  it  bet- 
ter by  making  use  of  the  press  1 

T.  I  might ;  but  I  was  willing  to  shew  you  how 
the  thing  might  be  done  if  no  such  apparatus  as  this 
were  at  hand.  The  use  of  the  table  and  press,  which, 
in  fact,  always  go  together,  is  for  keeping  steady  all 
descriptions  of  bodies,  through  which  the  charge  of  a 
single  jar,  or  any  number  of  which  a  battery  consists, 
is  to  be  conveyed.  We  will  now  proceed  with  the 
experiments. 

Ex.  6.  I  will  take  the  knobs  from  the  wires  of 
the  universal  discharger,  and  having  laid  a  piece  of 
very  dry  writing-paper  on  the  table  e,  I  place  the 
points  of  the  wires  at  an  inch  or  more  from  one 
another ;  then,  by  connecting  one  of  the  rings  c  with 
the  outside  wire  or  hook  of  the  battery,  and  bringing 
the  discharging  rod  from  the  other  ring  c  to  one  of 
the  knobs  of  the  battery,  you  will  see  that  the  paper 
will  be  torn  to  pieces. 

Ex.  7.  The  experiment  which  I  am  now  going  to 
make,  you  must  never  attempt  by  yourselves  :  I  put 
a  little  gunpowder  in  the  tube  of  a  quill,  open  at  both 
ends,  and  insert  the  pointed  extremities  of  the  two 
wires  in  it  so  as  to  be  within  a  quarter  of  an  inch  or 
less  from  each  other.  I  now  send  the  charge  of  the 
battery  through  it,  and  the  gunpowder,  you  see,  is  in- 
stantly inflamed. 

Ex.  8.  Here  is  a  very  slender  wire,  not  a  hun- 
dredth part  of  an  inch  in  diameter,  which  I  connect 
with  the  wires  of  the  discharger,  and  send  the  charge 
of  a  battery  through  it,  which  will  completely  melt  it, 
and  you  now  perceive  the  little  globules  of  iron  in- 
stead of  the  thin  wire. 


EXPERIMENTS. 


467 


C.  Will  other  wires  besides  iron  be  melted  in  the 
same  manner  1 

T.  Yes  ;  if  the  battery  be  large  enough,  and  the 
wires  sufficiently  thin,  the  experiment  will  succeed 
with  them  all ;  even  with  a  single  jar,  if  it  be  pretty 
large,  very  slender  wire  may  be  fused.  But  the 
charges  of  batteries  have  been  used  to  determine  the 
differeBt  conducting  powers  of  the  several  metals. 

J.  If  the  charge  is  not  strong  enough  to  melt  the 
wire,  will  it  make  it  red  hot  ? 

T.  It  will :  and  when  the  experiment  is  properly 
done,  the  course  of  the  fluid  may  be  discerned  by  its 
effects  :  for  if  the  wire  is  about  three  inches  long,  it 
will  be  seen  that  the  end  of  it  which  is  connected 
with  the  inside  of  the  battery,  is  red  hot  first,  and  the 
redness  proceeds  towards  the  other. 

C.  That  is  a  clear  proof  that  the  superabundant 
electricity  accumulated  in  the  inside  is  carried  to  the 
outside  of  the  jars. 

T,  Ex.  9.  We  have  already  discussed  the  subject 
of  magnetism :  by  discharging  the  battery  through 
a  small  sewing-needle,  it  will  become  magnetic  ; 
that  is,  if  the  needle  be  accurately  suspended  on  a 
small  piece  of  cork  in  a  bason  of  water,  one  end  will, 
of  itself,  point  to  the  north,  and  the  other  to  the 
south. 

Ex.  10.  I  will  lay  this  chain  on  a  sheet  of  writing- 
paper,  and  send  the  charge  of  the  battery  through  the 
chain  ;  and  you  will  see  black  marks  will  be  left  on 
the  paper  in  those  places  where  the  rings  of  the  chain 
touch  each  other. 

Ex.  10.  Place  a  small  piece  of  very  dry  wood 
between  the  balls  of  the  universal  discharger,  so  that 
the  fibres  of  the  wood  may  be  in  the  direction  of  the 
wires,  and  pass  the  charge  of  the  battery  through 
them,  and  the  wood  will  be  torn  in  pieces.  The 
points  of  the  wires  being  run  into  the  wood,  and  the 
shock  passed  through  them,  will  effect  the  same  thing. 

Ex.  12.  Here  is  a  glass  tube,  open  at  both  ends, 
six  inches  long,  and  a  quarter  of  an  inch  in  diameter. 


4G8 


ELECTRICITY. 


These  pieces  of  coik,  with  wires  in  them,  exactly  fit 
the  ends  of  the  tube.  I  put  in  one  cork,  and  fill  the 
tube  with  water,  then  put  the  other  cork  in,  and  push 
the  wires  so  that  they  nearly  touch,  and  pass  the 
charge  of  the  battery  through  them  ;  you  see  the  tube 
is  broken,  and  the  water  dispersed  in  every  direc- 
tion.* 

C.  If  water  is  a  good  conductor,  how  is  it  that  the 
charge  did  not  run  through  it  without  breaking  the 
tube? 

T.  The  electric  fluid,  like  common  fire,  converts 
the  water  into  a  highly  elastic  vapour,  which,  occu- 
pying very  suddenly  a  much  larger  space  than  the 
.water,  bursts  the  tube  before  it  can  effect  any  means 
of  escape. 

When  a  succession  of  electric  discharges  from  a 
powerful  electrical  machine  are  sent  through  water,  a 
decomposition  of  that  fluid  takes  place,  and  it  is  re- 
solved into  its  two  elements  of  oxygen  and  hydrogen, 
which  immediately  assume  the  gaseous  form. 


CONVERSATION  X. 

OF   THE    ELECTRIC    SPARK,    AND  MISCELLANEOUS 
EXPERIMENTS. 

T.  I  wish  you  to  observe  some  facts  connected 
with  the  electric  spark.  By  means  of  the  wire  in- 
serted in  this  ball,  I  fix  it  to  the  end  of  the  conductor, 
and  bring  either  another  brass  ball  or  my  knuckle  to 
it,  and  if  the  machine  act  pretty  powerfully,  a  long, 
crooked,  brilliant  spark  will  pass  between  the  two 
balls,  or  between  the  knuckle  and  ball.  If  the  con- 
ductor is  negative,  it  receives  the  spark  from  the 

*  To  prevent  accidents,  a  wire  cage,  such  as  is  used 
in  some  experiments  on  the  air-pump,  should  be  put 
over  the  tube  before  the  discharge  is  made ;  young  per- 
sons should  not  attempt  this  experiment  by  themselves. 


EXPERIMENTS. 


body  ;  but  if  it  is  positive,  the  ball  or  the  knuckle  re- 
ceives the  spark  from  the  conductor. 

C.  Does  the  size  of  the  spark  depend  at  all  on  the 
size  of  the  conductor  1 

T,  The  longest  and  largest  sparks  are  obtained 
from  a  large  conductor,  provided  the  machine  act 
very  powerfully.  When  the  quantity  of  electricity  is 
small,  the  spark  is  straight;  but  when  it  is  strong, 
and  capable  of  striking  at  a  greater  distance,  it  as- 
sumes what  is  called  a  zig-zag  direction. 

J.  If  the  electric  fluid  is  fire,  why  does  not  the 
spark,  which  excites  a  painful  sensation,  burn  me, 
when  I  receive  it  on  my  hand  ? 

T.  Ex.  1.  I  have  shewn  you  that  the  charge  from 
a  battery  will  make  iron  wire  red  hot,  and  inflame 
gunpowder.  Now  stand  on  the  stool  with  glass  legs, 
and  hold  the  chain  from  the  conductor  with  one  hand. 
Do  you,  Charles,  hold  this  spoon,  which  contains 
some  spirit  of  wine,  to  your  brother,  while  I  turn  the 
machine,  and  a  spark  taken  from  his  knuckle,  if  large, 
will  set  fire  to  the  spirit. 

C.  It  has  indeed :  did  you  do  nothing  with  the 
spirit  ? 

T.  I  only  made  the  silver  spoon  pretty  warm  before 
I  put  the  spirit  into  it. 

Ex.  2.  If  a  ball  of  box-wood  be  placed  on  the 
conductor  instead  of  the  brass  ball,  a  spark  taken 
from  it  will  be  of  a  fine  red  colour. 

Ex.  3.  An  ivory  ball  placed  on  the  conductor  will 
be  rendered  very  beautiful  and  luminous  if  a  strong- 
spark  be  taken  through  its  centre. 

Ex.  4.  Sparks  taken  over  a  piece  of  silver  leather 
appear  of  a  green  colour,  and  over  gilt  leather  of  a 
red  colour. 

Ex.  5.  Here  is  a  glass  tube,  round  which,  at  small 


470  ELECTRICITY, 
distances  from  each  other,  pieces  of  tin-foil  are  pasted 
in  a  spiral  form  from  end  to  end  ;  this  tube  is  inclosed 
in  a  larger  one  fitted  with  brass  caps  at  each  end, 
which  are  connected  with  the  tin-foil  of  the  inner 
tube. — I  hold  one  end  a  in  my  hand,  and  while  one 
of  you  turn  the  machine,  I  will  present  the  other  end 
B  to  the  conductor,  to  take  sparks  from  it :  but  first 
shut  the  window-shutters. 

C.  This  is  a  very  beautiful  experiment. 

T.  The  beauty  of  it  consists  in  the  distance  which 
is  left  between  the  pieces  of  tin-foil,  and  by  increasing 
the  number  of  these  distances,  the  brilliancy  is  very 
much  heightened. 

Ex.  6.  The  following  is  another  experiment  of  the 
same  kind :  here  is  a  word,  with  which  you  are  ac- 


Fig.  14. 


quainted,  made  on  glass,  by  means  of  tin-foil  pasted 
on  glass,  fixed  in  a  frame  of  baked  wood.  I  hold 
the  frame  in  my  hand  at  h,  and  present  the  ball  g  to 
the  conductor,  and  at  every  considerable  spark  the 
word  is  beautifully  illuminated. 

Ex.  7.  A  piece  of  sponge  fi'lled  with  water,  and 
hung  to  a  conductor,  when  electrified  in  a  dark  room, 
exhibits  a  beautiful  appearance. 

Ex.  8.  This  bottle  is  charged  :  if  I  bring  the  brass 
knob  that  stands  out  of  it  to  a  basin  of  water  which  is 
insulated,  it  will  attract  a  drop ;  and  on  the  removal 
of  the  bottle  it  will  assume  a  conical  shape,  and  if 
brought  near  any  conducting  substance,  it  will  fly  to 
it  in  luminous  streams. 

Ex. '9.  Place  a  drop  of  water  on  the  conductor, 
and  work  the  machine ;  the  drop  will  afford  a  long 
spark,  assume  a  conical  figure,  and  carry  some  of 
the  water  with  it. 


EXPERIMENTS.  47I 
Ex.  10.  On  this  wire  I  have  fixed  a  piece  of 
seahng-wax,  and  having  fixed  the  wire  into  the  end  of 
the  conductor,  I  will  light  the  wax,  and  the  moment 
the  machme  is  worked  the  wax  will  fly  oflT  in  the  finest 
filaments  imaginable. 

Ex.  11.  I  will  wrap  some  cotton-wool  round  one 
of  the  knobs  of  my  discharging  rod,  and  fill  the  wool 
with  finely  bruised  resin ;  I  now  discharge  a  Leyden 
jar,  or  a  battery,  in  the  common  way,  and  the  wool  is 
instantly  m  a  blaze.  The  covered  knob  must  touch 
the  knob  of  the  jar,  and  the  discharge  should  be 
efl'ected  as  quickly  as  possible. 

You  will  remember,  that  the  electric  fluid  always 
chooses  the  nearest  road,  and  the  best  conductors  to 
travel  by ;  in  proof  of  which  take  the  following  ex- 
periment : — 

Ex.  12.    With  this  chain  I  make  a  sort  of  W;  the 
wire  u  touches  the  outside  of  a  charged 
jar,  and  the  wire  x  is  brought  to  the    ^  ^^v* 
knob  of  the  jar,  and  in  the  dark  a  bril-      \  /\  f 
liant  W  is  visible.    But  if  the  wire  u       \  /  \  / 
is  continued  to  m,  the  electric  fluid  ° 
takes  a  shorter  road  to  x,  and,  of  course,     Fio-.  15. 
only  half  of  the  VV  is  seen,  viz.  that  " 
part  marked  mz  y:  but  if,  instead  of  the  wire  n  m, 
a  dry  stick  be  laid  in  its  place,  the  electric  matter 
will  prefer  a  longer  circuit,  rather  than  go  through 
a  bad  conductor,  and  the  whole  W  will  be  illu- 
minated. 

Ex.  13.  Here  is  a  two  ounce  phial,  half  full  of 
salad  oil;  through  the  cork  is  passed  a  piece  of 
slender  wire,  the  end  of  which,  within  the  phial,  is  so 
bent  as  to  touch  the  glass  just  below  the  surface  of 
the  oil.  I  place  my  thumb  opposite  the  point  of  the 
wire  in  the  bottle,  and  in  that  position  take  a  spark 
from  the  charged  conductor.  You  observe  that  the 
spark,  to  get  to  my  thumb,  has  actually  perforated 
the  glass.  In  the  same  way  I  can  make  holes  all 
round  the  phial. 


472  -  ELECTRICITY. 

C.  Would  the  experiment  succeed  with  water 
instead  of  oil  ? 

T.  No,  it  would  not. 

J.  At  any  rate  we  see  the  course  of  the  electric 
fluid  in  this  experiment,  for  the  spark  comes  from  the 
conductor  down  the  wire,  and  through  the  glass  to  the 
thumb. 

r.  Its  direction  is,  however,  better  shewn  in  this 
way : — 

Ex.  14.  At  that  end  of  the  conductor  which  is 
farthest  from  the  machine,  I  fix  a  brass  wire  about 
six  inches  long,  having  a  small  brass  ball  on  its  ex- 
tremity. To  this  ball,  when  the  machine  is  at  work, 
I  hold  the  flame  of  a  wax  taper. 

C.  The  flame  is  evidently  blown  from  the  ball  in 
the  direction  of  the  electric  fluid  ;  it  has  a  similar  ef- 
fect to  the  blast  of  a  pair  of  bellows. 

Ex.  15.  I  will  fix  a  pointed  wire  upon  the  prime 
conductor,  with  the  point  outward,  and  another  like 
wire  upon  the  insulated  rubber  :  shut  the  window- 
shutter,  and  I  will  work  the  machine  : — now  observe 
the  points  of  the  two  wires. 

J.  They  both  are  illuminated,  but  diflPerently. 
The  point  on  the  conductor  sends  out  a  sort  of  brush 
of  fire,  but  that  on  the  rubber  is  illuminated  with  a  star. 

r.  You  see,  then,  the  difference  between  the 
positive  and  negative  electricity.  The  appearance 
of  a  star  on  the  point  of  the  wire  will  shew  that 
the  electricity  is  positive ;  while,  on  the  contrary,  a 
luminous  brush  indicates  that  the  electricity  is 
negative. 

CONVERSATION  XI. 

MISCELLANEOUS    EXPERIMENTS           OF    THE  ELECTRO- 

PHORUS   ^  OF     THE     ELECTROMETER   AND  THE 

THUNDER  HOUSE. 

T.  I  shall  proceed  this  morning  with  some  other 
experiments  on  the  electrical  machine. 


EXPERIMENTS.  473 
Ex.  1.  Here  are  two  wires,  one  of  which  is  con- 
nected with  the  outside  of  this  charged  Leyden  jar, 
the  other  is  so  bent  as  easily  to  touch  the  knob  of  the 
jar.  The  two  straight  ends  I  bring  within  the  dis- 
tance of  the  tenth  of  an  inch  of  one  another,  and 
press  them  down  with  my  thumb,  and  in  this  position, 
having  darkened  the  room,  I  discharge  the  jar :  do 
you  look  upon  my  thumb. 

C.  It  was  so  transparent  that  I  think  I  even  saw 
the  bone  of  the  thumb ;  but  did  it  not  hurt  vou  very 
much  ?  ^ 
T,  With  attention,  you  might  observe  the  principal 
blood-vessels,  I  believe,  and  the  only  inconvenience 
that  I  felt  was  a  sort  of  tremor  in  my  thumb,  which 
IS  by  no  means  painful.  Had  the  wires  been  at 
double  the  distance,  the  shock  would  have  probably 
made  my  thumb  the  circuit,  which  must  have  caused 
a  more  powerful  and  unpleasant  sensation,  but  being 
so  close,  the  electric  fluid  leaped  from  one  wire  to  the 
other,  and  during  this  passage  it  illuminated  my 
thumb,  but  did  not  go  through  it. 

Ex.  2.  If,  instead  of  my  thumb,  a  decanter  full 
of  water,  having  a  flat  bottom,  were  placed  on  the 
wires,  and  the  discharge  made,  the  whole  of  the 
water  will  be  beautifully  illuminated. 

Ex.  3.  This  small  pewter  bucket  is  full  of  water, 
and  I  suspend  it  from  the  prime  conductor,  and  put 
m  a  glass  syphon,  with  a  bore  so  narrow  that  the 
water^will  hardly  drop  out.  See  what  will  happen 
when  I  work  the  machine ;  but  first  make  the  room 
dark. 

J.  It  runs  now  in  a  full  stream,  or  rather  in  several 
streams,  all  of  which  are  illuminated. 

T.  Ex.  4.    If  the  knob  a  commu-  ^p^i^^ 
nicate  with  the  outside  of  a  charged      ^  ^.B  \ 
Leyden  jar,  and  the  knob  b  with  the  'jj 
inside  coating,  and  each  be  held  about  ^ 
two  inches  from  the  lighted  candle        Fig.  16. 
and  opposite  to  one  another,  the  flame 
will  spread  towards  each,  and  a  discharge  will  be 


474  ELECTRICITY, 
made  through  it:  this  shews  tlie  conducting  power  of 
flame. 

This  instrument,  which  consists  of  two 
circular  plates,  of  which  the  largest  b  is 
about  fifteen  inches  in  diameter,  and  the 
other  A  14  inches,  is  called  an  electropho- 
rus.  The  under  plate  b  is  made  of  glass,  or 
sealing-wax,  or  of  any  other  non-con-  Fig.  17. 
ducting  substance ;  1  have  made  one 
with  a  mixture  of  pitch  and  chalk  boiled  together. 
The  upper  plate  a  is  sometimes  made  of  brass,  and 
sometimes  of  tin-plate,  but  this  is  of  wood,  covered 
very  neatly  with  tin -foil :  x  is  a  glass  handle  fixed  to 
a  socket,  by  which  the  upper  plate  is  removed  from 
the  under  one. 

C.  What  do  you  mean  by  an  electrophorus  ? 

T.  It  is,  in  fact,  a  sort  of  simple  electrical  machine, 
and  is  thus  used.  Rub  the  lower  plate  b  with  a  fine 
piece  of  new  flannel,  or  with  rabbits',  or  hares',  or 
cats'  skin,  and  when  it  is  well  excited,  place  upon  it 
the  upper  plate  a,  and  put  your  finger  on  the  upper 
plate  ;  then  remove  this  plate  by  the  glass  handle  ^, 
and  if  you  apply  your  knuckle,  or  the  knob  of  a 
coated  jar,  you  will  obtain  a  spark.  This  operation 
may  be  repeated  many  times  without  exciting  again 
the  under  plate. 

J.  Can  you  charge  a  Ley  den  jar  in  this  way? 

T.  Yes,  it  has  been  done,  and  by  a  single  excita- 
tion, so  as  to  pierce  a  hole  through  a  card. 

Here  is  another  kind  of  electrometer, 
which  is  by  far  the  most  sensible  that  has  | 
been  yet  invented ;  that  is,  it  is  capable  of  ^ 
discovering   the   smallest  quantities  of    ^  {  |x 
electricity,    a  is  a  glass  jar,  b  the  cover      1 1  ^\ 
of  metal,  to  which  is  attached  two  pieces 
of  gold  leaf  x,  or  two  pith-balls  sus- 
pended  on  threads  ;  on  the  sides  of  the 
glass  jar  are  two  narrow  strips  of  tin-foil. 

C.  How  is  this  instrument  used  ?  Fig.  18. 

2\  Any  thing  that  is  electrified  is  to  be 


LIGHTNING.  475 
brought  to  the  cover,  which  will  cause  the  pieces  of 
gold  leaf,  or  pith-balls,  to  diverge ;  and  the  sen- 
sibility of  this  instrument  is  so  great,  that  the  brush  of 
a  feather,  the  throwing  of  chalk,  hair-powder,  or 
dust,  against  the  cap  b,  evinces  strong  signs  of 
electricity. 

Ex.  5.  Place  on  the  cap  b  a  little  pewter,  or  any 
other  metallic  cup,  having  some  water  in  it ;  then 
take  from  the  fire  a  live  cinder,  and  put  it  in  the  cup, 
and  the  electricity  of  vapour  is  very  admirably  ex- 
hibited. 

A  thunder-cloud  passing  over  this  instrument  will 
cause  the  gold  leaf  to  strike  the  sides  at  every  flash 
of  lightning. 

Ex.  6.  I  will  excite  this  stick  of  sealing-wax,  and 
bring  it  to  the  cover  b — you  see  how  often  it  causes 
the  gold  leaf  to  strike  against  the  sides  of  the  glass. 

J .  Are  the  slips  of  tin-foil  intended  to  carry  away 
the  electric  fluid  communicated  by  the  objects  pre- 
sented to  the  cap  b  ? 

T,  They  are  j  and  by  them  the  equilibrium  is  re- 
stored. 


CONVERSATION  XII. 

\  OF  ATMOSPHERICAL  ELECTRICITY. 

C.  You  said,  yesterday,  that  the  electrometer  was 
affected  by  thunder  and  lightning  :  are  lightning  and 
'    electricity  similar  ? 

T.  They  are,  undoubtedly,  the  same  fluid  ;  and 
that  they  are  the  same  was  discovered  by  Dr.  Franklin, 
in  June,  1752. 

/.  How  did  he  ascertain  this  fact  1 
!  T.  He  was  led  to  form  the  theory,  from  observing 
the  power  which  uninsulated  yoints  have  in  drawing 
off  the  electricity  from  bodies.  And  having  made  his 
system,  he  was  waiting  for  the  erection  of  a  spire,  in 
Philadelphia,  to  carry  his  views  into  execution,  when 
I    it  occurred  to  him  that  a  boy's  kite  would  answer  his 

r 

i. 


47G 


ELECTRICITY. 


purpose  better  than  a  spire.  He  therefore  prepared  a 
kite,  and  having  raised  it,  he  tied  to  the  end  of  the 
string  a  silken  cord,  by  which  the  kite  was  completely 
insulated.  At  the  junction  of  the  two  strings  he 
fastened  a  key,  as  a  good  conductor,  in  order  to  take 
sparks  from  it. 

C.  Did  he  obtain  any  sparks  1 
T.  One  cloud,  which  appeared  like  a  thunder- 
cloud, passed  without  any  effect;  shortly  after,  the 
loose  threads  of  the  hempen  string  stood  erect,  in  the 
same  manner  as  they  would  if  the  string  had  been 
hung  on  an  electrified  insulated  conductor.  He 
then  presented  his  knuckle  to  the  key,  and  obtained 
an  evident  spark.  Others  succeeded  before  the 
string  was  wet,  but  when  the  rain  had  wetted  the 
string,  he  collected  the  electricity  very  plentifully. 
J.  Could  I  do  so  with  our  large  kite  1 
T.  I  hope  you  will  not  try  to  raise  your  kite  during 
a  thunder-storm,  because,  without  very  great  care,  it 
may  be  attended  with  the  most  serious  danger ;  your 
kite  is,  however,  quite  large  enough,  being  four  feet 
high,  and  two  feet  wide;  every  thing  depends  on  the 
string,  which,  according  to  Mr.  Cavallo,  who  has 
made  many  experiments  on  the  ^subject,  should  be 
made  of  two  thin  threads  of  twine,  twisted  with  a 
copper  thread.  And  to  Mr.  Cavallo's  work  on  elec- 
tricity, vol.  2,  such  persons  as  are  desirous  of  raising 
kites,  for  electrical  purposes,  should  be  referred,  in 
which  they  will  find  ample  instruction. 

C.  How  do  the  conductors,  which  I  have  seen  fixed 
to  various  buildings,  act  in  dispersing  lightning  ? 

r.  You  know  how  easy  it  is  to  charge  a  Leyden 
jar  :  but  if,  when  the  machine  is  at  work,  a  person 
hold  a  point  of  steel,  or  other  metal,  near  the  con- 
ductor, the  greater  part  of  the  fluid  will  run  away  by 
that  point  instead  of  proceeding  to  the  jar.  Hence  it 
was  concluded,  that  pointed  rods  would  silently  draw 
away  the  lightning  from  clouds  passing  over  any 
building. 

J,  Is  there  not  a  particular  method  of  fixing  them? 


LIGHTNING.  477 

Yes  :  the  metallic  rod  must  reach  from  the 
ground,  or  the  nearest  piece  of  water,  to  a  foot  or 
two  above  the  building  it  is  intended  to  protect,  and 
the  iron  rod  should  come  to  a  fine  point :  some  elec- 
tricians recommend  that  the  point  should  be  of  gold, 
to  prevent  its  rusting. 

C.  What  effects  would  be  produced  if  lightning 
should  strike  a  building  without  a  conductor  ? 

T,  That  may  be  best  explained  by  informing  you 
of  what  happened,  many  years  ago,  to  St.  Bride's 
church.  The  lightning  first  struck  the  weather-cock : 
from  thence,  descending  in  its  progress,  it  beat  out  a 
number  of  large  stones  of  different  heights,  some  of 
which  fell  upon  the  roof  of  the  church,  and  did 
great  damage  to  it.  The  mischief  done  to  the  steeple 
was  so  considerable,  that  eighty-five  feet  of  it  was 
obliged  to  be  taken  down. 

/ .  The  weather-cock  was  probably  made  of  iron  j 
why  did  not  that  act  as  a  conductor  ] 

T,  Though  that  was  made  of  iron,  yet  it  was  com- 
pletely insulated  by  being  fixed  in  stone,  that  had  be- 
come dry  by  much  hot  and  dry  weather.  When 
therefore  the  lightning  had  taken  possession  of  the 
weather-cock,  by  endeavouring  to  force  its  way  to 
another  conductor,  it  beat  down  whatever  stood  in  its 
way. 

C.  The  power  of  lightning  must  be  very  great. 

T.  It  is  irresistible  in  its  effects  ;  the  following 
experiment  will  illustrate  what  I  have  been  saying 

Ex.  1.  A  is  a  board  representing 
the  gable  end  of  a  house ;  it  is 
fixed  on  another  board  b  :  a  b  c  d  is 
a  square  hole,  to  which  a  piece  of 
wood  is  fitted  ;  a  d  represents  a  wire 
fixed  diagonally  on  the  wood  a  b 
c  d ;  X  b,  terminated  by  a  knob  x, 
represents  a  weather-cock,  and  the 
wire  c  z  is  fixed  to  the  board  a. 

It  is  evident,  that  in  the  state  in 
which  it  is  drawn  in  the  figure,  there         Fig.  19. 


478  ELECTRICITY, 
is  an  interruption  in  the  conducting  rod ;  accordingly, 
if  the  chain  m  is  connected  with  the  outside  of  a 
Ley  den  phial,  and  then  that  phial  is  discharged 
through  X,  by  bringing  one  part  of  the  discharging 
rod  to  the  knob  of  the  Leyden  phial,  and  the  other 
to  within  an  inch  or  two  of  x,  the  piece  of  wood, 
a,  6,  c,  d,  will  be  thrown  out  with  violence. 

J.  Are  we  to  understand  by  this  experiment  that  if 
the  wire  x  h  had  been  continued  to  the  chain,  that  the 
electric  fluid  would  have  run  through  it  without  dis- 
turbing the  loose  board  1 

T.  Ex.  2.  Just  so  ;  for  if  the  piece  of  wood  be 
taken  out,  and  the  part  a  be  put  to  the  place  b,  then 
d  will  come  to  c,  and  the  conducting  rod  will  be  com- 
plete, and  continued  from  x  through  a  and  d  to  z,  and 
now  the  phial  may  be  discharged  as  often  as  you 
please,  but  the  wood  will  remain  in  its  place,  because 
the  electric  fluid  runs  thiough  the  wire  to  z,  and 
makes  its  way  by  the  chain  to  the  outside  of  the 
phial. 

C.  Then  if  x  be  supposed  the  weather-cock  of  the 
church,  the  lightning  having  overcharged  this,  by  its 
endeavours  to  reach  another  conductor,  as  c  s, 
forced  away  the  stone  or  stones  represented  by  a  6 
c  d  ? 

T.  That  is  what  1  meant  to  convey  to  your  minds 
by  the  first  experiment ;  and  the  second  shews  very 
clearly,  that  if  an  iron  rod  had  gone  from  the 
weather-cock  to  the  ground,  without  interruption,  it 
would  have  conducted  away  the  electricity  silently, 
and  without  doing  any  injury  to  the  church. 

J.  How  was  it  that  all  the  stones  were  not  beat 
down  ? 

T,  Because,  in  its  passage  downwards,  it  met  with 
many  other  conductors.  I  will  read  part  of  what 
Dr.  Watson  says  on  this  fact,  who  examined  it 
very  attentively  : —  j 

"The  lightning,"  says  he,  "first  took  a  weather- 
cock, which  was  fixed  at  the  top  of  the  steeple,  and 
was  conducted  without  injuring  the  metal  or  anything 


FALLING  STARS.  479 
else  as  low  as  where  the  large  iron  bar  or  spindle 
which  supported  it  terminated :  there  the  metallic 
communication  ceasing,  part  of  the  lightning  ex- 
ploded, cracked,  and  shattered  the  obelisk,  which  ter- 
minated the  spire  of  the  steeple,  in  its  whole  diameter, 
and  threw  off,  at  that  place,  several  large  pieces  of 
Portland  stone.  Here  it  likewise  removed  a  stone 
from  its  place,  but  not  far  enough  to  be  thrown  down. 
From  thence  the  lightning  seemed  to  have  rushed 
upon  two  horizontal  iron  bars,  which  were  placed 
within  the  building  across  each  other.  At  the  end 
of  one  of  these  iron  bars  it  exploded  again,  and 
threw  off  a  considerable  quantity  of  stone.  Almost 
all  the  damage  was  done  where  the  ends  of  the  iron 
bars  had  been  inserted  into  the  stone,  or  placed  under 
it;  and,  in  some  places,  its  passage  might  be  traced 
from  one  iron  bar  to  another." 


CONVERSATION  XIII. 

ON   ATMOSPHEinC   ELECTRICITY  OF  FALLING  STARS— 

OF  THE  AURORA  BOREALIS  OF  WATER-SPOUTS  AND 

WHIRLWINDS  OF  EARTHQUAKES. 

C.  Does  the  air  always  contain  electricity  1 

T.  Yes  ;  and  it  is  owing  to  the  electricity  of  the 
atmosphere  that  we  observe  a  number  of  curious  and 
interesting  phenomena,  such  as  falling  stars  ;  the 
aurora  borealis,  or  northern  lights ;  the  ignis  fatuus, 
or  Will-with-the-wisp. 

J .  I  have  frequently  seen  what  people  call  falling 
stars,  but  I  never  knew  that  they  were  occasioned 
merely  by  electricity. 

T.  These  are  seen  chiefly  in  clear  and  calm 
weather  :  it  is  then  that  the  electric  fluid  is  probably 
not  very  strong,  and,  passing  through  the  air,  it  be- 
comes visible  in  particular  parts  of  its  passage,  ac- 
cording to  the  conducting  substances  it  may  meet 
with.  One  of  the  most  striking  phenomena  of  this  kind 


480 


ELECTRICITY. 


is  recorded  by  Signior  Beccaria  : — As  he  was  sitting 
with  a  friend  in  the  open  air,  an  hour  after  sun-set, 
they  saw  a  falUng,  or,  as  it  is  sometimes  called,  a 
shooting  star,  directing  its  course  towards  them,  grow- 
ing, apparently,  larger  and  larger,  till  it  disappeared 
not  far  from  them,  and,  disappearing,  it  left  their 
faces,  hands,  and  clothes,  with  the  earth  and  neigh- 
bouring objects,  suddenly  illuminated  with  a  diffused 
and  lambent  light,  attended  with  no  noise  at  all. 

C.  But  how  did  he  know  that  this  was  only  the 
effect  of  electricity  1 

T.  Because  he  had  previously  raised  his  kite,  and 
found  the  air  very  much  charged  with  the  electric 
matter  :  sometimes  he  saw  it  advancing  to  his  kite  like 
a  falling  star  ;  and  sometimes  he  saw  a  kind  of  glory 
round  it,  which  followed  it  as  it  changed  its  place. 

/.  Since  lofty  objects  are  exposed  to  the  effects  of 
lightning,  or  the  electric  fluid,  do  not  the  tail  masts 
of  ships  run  considerable  risk  of  being  struck  by  it  ? 

T.  Certainly  :  we  have  many  instances  recorded 
of  the  mischief  done  to  ships.  One  of  which  is  re- 
lated in  the  Philosophical  Transactions  ;  it  happened 
on  board  the  Montague,  on  the  4th  of  November, 
1748,  in  lat.  42"  48'  and  9°  3'  west  longitude,  about 
noon.  One  of  the  quarter-masters  desired  the  master 
of  the  vessel  to  look  to  the  windward,  when  he  ob- 
served a  large  ball  of  blue  fire,  rolling  apparently  on 
the  surface  of  the  water,  at  the  distance  of  three  miles 
from  them  :  it  rose  almost  perpendicular  when  it  was 
within  forty  or  fifty  yards  from  the  main-chains  of 
the  ship,  it  then  went  off  with  an  explosion,  as  if  a 
hundred  cannon  had  been  fired  at  one  time,  and  left 
so  great  a  smell  of  sulphur,  that  the  ship  seemed  to 
contain  nothing  else.  After  the  noise  had  subsided, 
the  main-top-mast  was  found  shattered  to  pieces, 
and  the  mast  itself  was  rent  quite  down  to  the  keel. 
Five  men  were  knocked  down,  and  one  of  them 
greatly  burnt  by  the  explo^ijii. 

C.  Did  it  not  seem  to  be  a  very  large  ball  to  have 
produced  such  cfTects? 


AURORA  EOREALiS.  48i 

T.  Yes;  the  person  who  noticed  it  said  it  was  as 
Dig  as  a  millstone. 

The  Aurora  Borealis  is  another  electrical  pheno- 
menon :  this  IS  admitted  without  any  hesitation,  be- 
cause  electricians  can  readily  imitate  the  appearance 
with  their  experiments. 

sca^e      "^^^^        ^  ^^^"^^  ^  ^^^^  ^"""^^ 

T,  True  ;  there  is  a  glass  tube  about  thirty  inches 
long  and  the  diameter  of  it  is  about  two  inches  ;  it  is 
nearly  exhausted  of  air,  and  capped  on  both  ends 
with  brass.  I  now  connect  these  ends,  by  means  of 
a  Cham,  with  the  positive  and  negative  part  of  a  ma- 
chine,  and  in  a  darkened  room  you  will  see,  whe*n 
the  machine  is  worked,  all  the  appearances  of  the 
northern  lights  in  the  tube. 

C.  Why  is  it  necessary  nearly  to  exhaust  the  tube  ^ 
i .  Because  the  air,  in  its  natural  state,  is  a  very 
bad  conductor  of  the  electric  fluid  ;  but  when  it  is 
perhaps,  rendered  some  hundred  times  rarer  than  it 
usually  is,  the  electric  fluid  darts  from  one  cap  to 
the  other  with  the  greatest  ease. 

/.  But  we  see  the  natural  aurora  borealis  in  the 
air. 

T.  We  do  so,  but  it  is  in  the  higher  regions  of  the 
atmosphere,  where  the  air  is  much  rarer  than  it  is 
near  the  surface  of  the  earth.  The  experiment  which 
you  have  just  seen  accounts  for  the  darting  and  un- 
dulatmg  motion  which  takes  place  between  the  oppo- 
site  parts  of  the  heavens.  The  aurora  borealis  is  the 
most  beautiful  and  brilliant  in  countries  in  the  hioh 
northern  latitudes,  as  in  Greenland  and  Iceland.  ° 

The  aurora  borealis  that  was  seen  in  this  country 
on  the  2t3d  of  October,  1804,  is  deserving  of  notice 
At  seven  m  the  evening  a  luminous  arch  was  seen 
trom  the  centre  of  London,  extending  from  one  point 
ot  the  horizon,  about  s.s.  w.  to  another  point  n.n  w 
and  passing  the  middle  of  the  constellation  of  the 
Great  Bear,  which  it  in  a  great  measure  obscured. 
It  appeared  to  consist  of  shining  vapour,  and  to  roll 


482  ELECTRICITY, 
from  the  south  to  the  north.  In  about  half  an  hour 
its  course  was  changed  ;  it  then  became  vertical,  and 
about  nine  o'clock  it  extended  across  the  heavens 
from  N.  E.  to  s.  w. ;  at  intervals  the  continuity  of  the 
luminous  arch  was  broken,  and  there  then  darted 
from  its  south-west  quarter,  tov/ards  the  zenith,  strong 
flashes  and  streaks  of  bright  red,  similar  to  what 
appears  in  the  atmosphere  during  a  great  fire  in 
any  part  of  the  metropolis.  For  several  hours  the 
atmosphere  was  as  light  in  the  south-west  as  if  the 
sun  had  set  but  half  an  hour ;  and  the  light  in  the 
north  resembled  the  strong  twilight  which  marks  that 
part  of  the  horizon  at  midsummer. 

J.  How  do  you  account,  sir,  for  the  Will-with-the- 
wisp,  or  Jack-a-lanthorn,  that  is  close  to  the  ground, 
where  the  air  is  thickest  ? 

T.  This  is  a  meteor  which  seldom  appears  more 
than  six  feet  above  the  ground  ;  it  is  always  about 
bogs  and  swampy  places,  and  these,  in  hot  weather, 
emit  what  is  called  inflammable  air,  which  is  easily 
set  fire  to  by  the  electric  spark.  These,  therefore,  as 
you  shall  see  in  our  chemical  experiments,  we  can  as 
readily  imitate  as  the  aurora  borealis. — In  some  parts 
of  Italy  meteors  of  this  kind  are  frequently  very  large, 
and  give  a  light  equal  to  that  of  a  torch. 

Waterspouts,  which  are  sometimes  seen  at  sea,  are 
supposed  to  arise  from  the  power  of  electricity. 

C.  I  have  heard  of  these  ;  but  I  thought  that  ,i 
water-spouts  at  sea,  and  whirlwinds  and  hurricanes  ? 
by  land,  were  produced  solely  by  the  force  of  the 
wind.  . 

T.  The  wind  is,  undoubtedly,  one  of  the  causes,  but! 
it  will  not  account  for  every  appearance  connected 
with  them.  Water-spouts  are  often  seen  in  calm 
weather,  when  the  sea  seems  to  boil,  and  send  up  a 
smoke  under  them,  rising  in  a  sort  of  hill  towards  the 
spout.  A  rum.bling  noise  is  often  heard  at  the  time 
of  their  appearance,  which  happens  generally  in  those 
months  that  are  peculiarly  subject  to  thunder-storms, 
and  they  are  commonly  accompanied  or  followed  by 


WATERSPOUTS. 


483 


lightning.  When  these  approach  a  ship,  the  sailors 
present  and  brandish  their  swords  to  disperse  them, 
which  seems  to  favour  the  conclusion^  that  they  are 
electrical. 

J,  Do  the  swords  act  as  conductors  ? 

T.  They  may,  certainly  :  and  it  is  known  that  by 
these  pointed  instruments  they  have  been  efFectually 
dispersed. 

The  analogy  between  the  phenomena  of  water- 
spouts and  electricity,  may  be  made  visible  by  hang- 
ing a  drop  of  water  to  a  wire,  communicating  with  the 
prime  conductor,  and  placing  a  vessel  of  water  under 
it.  In  these  circumstances,  the  drop  assumes  all  the 
various  appearances  of  a  water-spout,  in  its  rise,  form, 
and  mode  of  disappearing. 

Water-spouts,  at  sea,  are  undoubtedly  very  like 
whirlwinds  and  hurricanes  by  land.  These  some- 
times tear  up  trees,  throw  down  buildings,  make 
caverns  ;  and  in  all  the  cases  they  scatter  the  earth, 
bricks,  stones,  timber,  &:c.  to  a  great  distance  in  every 
direction.  Dr.  Franklin  mentions  a  remarkable  ap- 
pearance which  occurred  to  Mr.  Wilkie,  a  consider- 
able electrician.  On  the  20th  of  July,  1758,  at 
three  o'clock  in  the  afternoon,  he  observed  a  great 
quantity  of  dust  rising  from  the  ground  and  cove^ring 
a  field  and  part  of  the  town  in  which  he  then  was. 
"There  was  no  wind,  and  the  dust  moved  gently  to- 
wards the  east,  where  there  appeared  a  great  black 
cloud,  which  electrified  his  apparatus  positively  to  a 
very  high  degree.  This  cloud  went  towards  the 
west,  the  dust  followed  it,  and  continued  to  rise 
higher  and  higher,  till  it  composed  a  thick  pillar,  in 
the  form  of  a  sugar-loaf,  and  at  length  it  seemed  to 
be  in  contact  with  the  cloud.  At  some  distance  from 
this,  there  came  another  great  cloud,  with  a  long 
stream  of  smaller  ones,  which  electrified  his  apparatus 
negatively,  and  when  they  came  near  the  positive 
cloud,  a  flash  of  lightning  was  seen  to  dart  through 
the  cloud  of  dust,  upon  which  the  negative  clouds 


48^1  ELECTRICITY, 
spread  very  much  and  dissolved  in  rain,  wbicn  pre- 
sently cleared  the  atmosphere. 

C.  Is  rain,  then,  an  electrical  phenomenon  I 

T.  The  most  enlightened  and  best  informed  elec- 
tricians reckon  rain,  hail,  and  snow,  among  the  effects 
produced  by  the  electric  fluid. 

J.  Do  the  negative  and  positive  clouds  act  in  the 
same  manner  as  the  outside  and  inside  coatings  of  a 
charged  Leyden  jar? 

T.  Thunder-clouds  frequently  do  nothing  more 
than  conduct  or  convey  the  electric  matter  from  one 
place  to  another. 

C.  Then  they  may  be  compared  to  the  discharging 

YOdl 

T.  And  perhaps,  like  that,  they  are  intended  to  re- 
store the  equilibrium  between  two  places,  one  of 
which  has  too  much,  and  the  other  too  little,  of  the 
electric  fluid.  The  following  is  not  an  uncommon 
appearance:  a  dark  cloud  is  observed  to  attract  others 
to  it,  and  when  grown  to  a  considerable  size,  its  lower 
surface  swells  in  particular  parts  towards  the  earth. 
During  the  time  that  the  cloud  is  thus  forming, 
flashes  of  lightning  dart  from  one  part  of  it  to  the 
other,  and  often  illuminate  the  whole  mass ;  and 
small  clouds  are  observed  moving  rapidly  beneath 
it.  When  the  cloud  has  acquired  a  sufficient  ex- 
tent, the  lightning  strikes  the  earth  in  two  opposite 
places. 

J.  I  wonder  the  discharge  does  not  shake  the 
earth,  as  the  charge  of  a  jar  does  anything  through 
which  it  passes. 

T.  Every  discharge  of  clouds  through  the  earth 
may  do  this,  though  it  is  imperceptible  to  us. 

Earthquakes  are  probably  occasioned  by  vast  dis- 
charges of  the  electric  fluid  :  they  happen  most  fre- 
quently in  dry  and  hot  countries,  which  are  subject 
to  lightning  and  other  electric  phenomena ;  they  are 
even  foretold  by  the  electric  corruscations,  and  other 
appearances  in  the  air  for  some  days  preceding  the 


MEDICAL  ELECTRICITY. 


485 


event.  Besides,  the  shoek  of  an  earthquake  is  in- 
stantaneous to  the  greatest  distances.  Earthquakes 
are  usually  accompanied  with  rain,  and  sometimes  by 
the  most  dreadful  thunder-storms. 


CONVERSATION  XIV. 

MEDICAL  ELECTRICITY. 

T.  If  you  stand  on  the  stool  with  glass  legS;,  and 
hold  the  chain  from  the  conductor  while  I  work  the 
machine  a  few  minutes,  your  pulse  will  be  increased, 
that  is,  it  will  beat  more  frequently  than  it  did  before. 
From  this  circumstance  physicians  have  applied  elec- 
tricity to  the  cure  of  many  disorders :  in  some  of 
which  their  endeavours  have  been  unavailing — in 
others  the  success  has  been  very  complete, 

C.  Did  they  do  nothing  more  than  this  1 

T,  Yes ;  in  some  cases  they  took  sparks  from 
their  patients — in  others  they  gave  them  shocks. 

/.  This  would  be  no  pleasant  method  of  cure,  if 
the  shocks  were  strong. 

T.  You  know  by  means  of  Lane's  electrometer, 
described  in  our  seventh  Conversation,  the  shock  may 
be  given  as  slightly  as  you  please. 

C.  But  how  are  shocks  conveyed  through  any 
part  of  the  body  ? 

T.  There  are  machines  and  apparatus  made  pur- 
posely for  medical  purposes,  but  every  end  may  be 
answered  by  the  instrument  just  referred  to.  Suppose 
the  electrometer  to  be  fixed  to  a  Leyden  phial,  and 
the  knob  at  a  to  touch  the  conductor,  and  the  knob 
B  to  be  so  far  off,  as  you  mean  the  shocks  to  be  weak 
or  strong,  one  chain  or  wire  is  to  be  fixed  to  the 
ring  c  of  the  electrometer,  and  another  wire  or  chain 
to  the  outside  coating  :  the  other  ends  of  these  two 
wires  are  to  be  fastened  to  the  two  knobs  of  the  dis- 
charging rod. 

J.  What  next  is  to  be  done,  if  I  wish  to  electrify 
my  knee,  for  instance  1 


488  ELECTRICITY. 

T.  All  you  have  to  do  is  to  bring  the  balls  of  the 
discharging  rod  close  to  your  knee,  one  on  the  one 
side,  and  the  other  on  the  opposite  side. 

C.  And,  at  every  discharge  of  the  Leyden  jar,  the 
superabundant  electricity  from  withinside  will  pass 
from  the  knob  at  a  to  the  knob  b,  and  will  pass 
through  the  wire  and  the  knee,  in  its  way  to  the  out- 
side of  the  jar,  to  restore  to  both  sides  an  equilibrium. 

J.  But  if  it  happen  that  a  part  of  the  body,  as  the 
arm,  is  to  be  electrified,  how  is  it  to  be  done,  because 
in  that  case  I  cannot  use  both  my  hands  in  conduct- 
ing the  wires  1 

T.  Then  you  may  seek  the  assistance  of  a  friend, 
who  will,  by  means  of  two  instruments  called  direc- 
tors, be  able  to  conduct  the  fluid  to  any  part  of  the 
body  whatever. 

C.  What  are  directors? 

T,  A  director  consists  of  a  knobbed  brass  wire, 
which  by  means  of  a  brass  cap  is  cemented  to  a  glass 
handle.  So  the  operator,  holding  these  directors  by 
the  extremities  of  the  glass  handle,  brings  the  balls,  to 
which  the  wires  or  chains  are  attached,  into  contact 
with  the  extremities  of  that  part  of  the  body  of  the 
patient  through  which  the  shock  is  to  be  sent.  If  I 
feel  rheumatic  pains  between  my  elbow  and  wrist, 
and  a  person  hold  one  director  at  the  elbow  and 
another  about  the  wrist,  the  shocks  will  pass  through, 
and  probably  will  be  found  useful  in  removing  the 
complaint, 

J.  Is  it  necessary  to  stand  on  the  glass-footed 
stool  to  have  this  operation  performed  ? 

T.  By  no  means  :  when  shocks  are  administered, 
the  person  who  receives  them  may  stand  as  he 
pleases,  either  on  the  stool  or  on  the  ground  ;  the 
electric  fluid,  taking  the  nearest  passage,  will  always 
find  the  other  knob  of  the  other  director,  which  leads 
to  the  outside  of  the  jar. 

C.  Is  it  necessary  to  make  the  body  bare? 

T.  Not  in  the  case  of  shocks,  unless  the  coverings 
be  very  thick  :  but  when  sparks  are  to  be  taken,  then 


THE  TORPEDO. 


48T 


the  person  from  whom  they  are  drawn  must  be  insu- 
lated, and  the  clothes  should  be  stripped  off  the  part 
affected. 

J.  For  what  disorders  are  the  shocks  and  sparks 
chiefly  used?  . 

r.  Shocks  have  been  found  useful  in  paralytic  dis- 
orders ;  in  contractions  of  the  nerves ;  in  sprains,  and 
in  many  other  cases  ;  but  great  attention  is  necessary 
in  regulating  the  force  of  the  shock,  because,  instead 
of  advantage,  mischief  may  occur  if  it  be  too  violent. 

C.  Is  there  less  danger  with  sparks  ? 

r.  Yes  ;  for  unless  it  be  in  very  tender  parts,  as 
the  eye,  there  is  no  great  risk  in  taking  sparks ;  and 
they  have  proved  very  effectual  in  removing  many 
complaints. 

The  celebrated  Mr.  Ferguson  was  seized,  at  Bristol, 
with  a  violent  sore  throat,  so  as  to  prevent  him  from 
swallowing  any  thing :  he  caused  sparks  to  be  taken 
from  the  part  affected,  and  in  the  course  of  an  hour 
he  could  eat  and  drink  without  pain. 

This,  in  some  instances,  has  been  found  an  excel- 
lent method  in  cases  of  deafness,  ear-ache,  tooth-ache, 
swellings  inside  the  mouth,  &c. 

J.  Would  not  strong  sparks  injure  the  ear  ? 

T.  They  might ;  and  therefore  the  electric  fluid  is 
usually  drawn  with  a  pointed  piece  of  wood,  to 
which  it  comes  in  a  stream,  or,  when  sparks  are  taken, 
a  very  small  brass  ball  is  used,  because,  in  propor- 
tion to  the  size  of  the  ball  is  the  size  of  the  spark. 


CONVERSATION  XV. 

OF  ANIMAL   ELECTRICITY  OF  THE  TORPEDO— OF  THE 

GYMNOTUS  ELECTRICUS  OF  THE  SILURUS  ELECTRICUS. 

T.  There  are  three  kinds  of  fish  which  have  been 
discovered,  that  are  possessed  of  the  singular  property 
of  giving  shocks  very  similar  to  those  experienced  by 
means  of  the  Leyden  jar. 


488 


ELECTRICITY. 


C.  I  should  like  much  to  see  them  ;  are  they 
easily  obtained  ? 

T.  No,  they  are  not :  they  are  called  the  torpedo, 
the  gymnotKS  electricus,  and  the  silurus  electricus, 

J.  Are  they  all  of  the  same  genus  1 

T,  No  ;  the  torpedo  is  a  flat  fish,  seldom  twenty 
inches  long,  and  is  common  in  various  parts  of  the 
sea  coast  of  Europe.  The  electric  organs  of  this  fish 
are  placed  on  each  side  of  the  gills,  where  they  fill  up 
tlie  whole  thickness  of  the  animal,  from  the  lower  to 
the  upper  surface,  and  are  covered  by  the  common 
skin  of  the  body, 

C.  Can  you  lay  hold  of  the  fish  by  any  other  part 
of  the  body  with  impunity  ? 

T.  Not  altogether  so  :  for  if  it  be  touched  with  one 
hand,  it  generally  communicates  a  very  slight  shock  ; 
but  if  it  be  touched  with  both  hands  at  the  same  time, 
one  being  applied  to  the  under,  and  the  other  to  the 
upper  surface  of  the  body^  a  shock  will  be  received 
similar  to  that  which  is  occasioned  by  the  Leyden  jar, 

J.  Will  not  the  shock  be  felt  if  both  hands  be  put 
on  one  of  the  electrical  organs  at  the  same  time  ? 

T.  No :  and  this  shews  that  the  upper  and  lower 
surfaces  of  the  electric  organs  are  in  opposite  states  of 
electricity,  answering  to  the  positive  and  negative 
sides  of  a  Leyden  phial. 

C.  Are  the  same  substances  conductors  of  the 
electric  power  of  the  torpedo,  by  which  artificial 
electricity  is  conducted  ? 

T.  Yes,  they  are  :  and  if  the  fish,  instead  of  being 
touched  by  the  hands,  be  touched  by  conducting 
substances,  as  metals,  the  shock  will  be  communicated 
through  them.  The  circuit  may  also  be  formed  by 
several  persons  joining  hands,  and  the  shock  will  be 
felt  by  them  all  at  the  same  time.  But  the  shock 
will  not  pass  where  there  is  the  smallest  interruption ; 
it  will  not  even  be  conducted  through  a  chain. 
J.  Can  you  get  sparks  from  it  1 
T.  No  spark  was  ever  obtained  from  the  torpedo. 


THE  GYMNOTUS. 


489 


nor  could  electric  repulsion  and  attraction  be  pro- 
duced by  it. 

C.  Is  it  known  how  the  power  is  accumulated  ? 

T.  It  seems  to  depend  on  the  will  of  the  animal, 
for  each  effort  is  accompanied  with  a  depression  of  its 
eyes,  and  it  probably  makes  use  of  it  as  a  means  of 
self-defence. 

J.  Is  this  the  case  also  with  the  other  electrical 
fishes  ? 

T,  The  gymnotus  possesses  all  the  electric  pro- 
perties of  the  torpedo,  but  in  a  very  superior  degree. 
This  fish  has  been  called  the  electrical  eel,  on  account 
of  its  resemblance  to  the  common  eel.  It  is  found 
in  the  large  rivers  in  South  America. 

C.  Are  these  fishes  able  to  injure  other  fishes  by 
this  power  1 

T,  If  small  fishes  are  put  into  the  water  in  which 
the  gymnotus  is  kept,  it  will  first  stun,  'or  perhaps  kill 
them,  and  if  the  animal  be  hungry,  it  will  then  de- 
vour them.  But  fishes  stunned  by  the  gymnotus 
may  be  recovered,  by  being  speedily  removed  into 
another  vessel  of  water. 

The  gymnotus  is  said  to  be  possessed  of  a  new 
kind  of  sense,  by  which  it  knows  whether  bodies  that 
are  brought  near  it  are  conductors  or  not. 

C.  Then  it  possesses  the  same  knowledge  by  in- 
stinct which  philosophers  have  gained  by  experiment. 

T.  You  are  right.  The  following  experiment, 
among  others,  is  very  decisive  on  this  point. 

Ex.  The  extremities  of  two  wires  were  dipped  into 
the  water  of  the  vessel  in  which  the  animal  was  kept ; 
they  were  then  bent,  extended  a  great  way,  and  ter- 
minated in  two  separate  glasses  full  of  water.  These 
wires,  being  supported  by  non-conductors,  at  a  con- 
siderable distance  from  each  other,  the  circuit  was 
incomplete  :  but  if  a  person  put  the  fingers  of  both 
hands  into  the  glasses  in  which  the  wires  terminated, 
then  the  circuit  was  complete.  While  the  circuit  was 
incomplete,  the  fish  never  went  near  the  extremities  of 
Y  2 


490 


ELECTRICITY. 


the  wires,  as  if  desirous  of  giving  the  shock  ;  but  the 
moment  the  circuit  was  completed,  either  by  a  person, 
or  any  other  conductor,  the  gymnotus  immediately 
went  towards  the  wires,  and  gave  the  shock,  though 
the  completion  of  the  circuit  was  out  of  his  sight. 

/.  How  do  they  catch  these  kinds  of  fish  ;  the  man 
would,  probably,  let  them  go  on  receiving  the  shock  1 
T,  In  this  way  the  property  was,  perhaps,  first 
discovered.  The  gymnotus,  as  well  as  the  others, 
may  be  touched,  without  any  risk  of  the  shock, 
with  wax  or  with  glass ;  but  if  it  be  touched  with 
the  naked  finger,  or  with  a  metal,  or  a  gold  ring, 
the  shock  is  felt  up  the  arm. 

C  Does  the  silurus  electricus  produce  the  same 
effects  as  the  others? 

T.  This  fish  is  found  in  some  rivers  in  Africa,  and 
it  is  known  to  possess  the  property  of  giving  the 
shock,  but  no  other  particulars  have  been  detailed 
respecting  it. 

With  regard  to  the  torpedo,  its  power  of  giving  the 
benumbing  sensation  was  known  to  the  ancients,  and 
from  this  it  probably  took  its  name.  In  Firmin^s 
Natural  History  of  Surinam  is  some  account  of  the 
tremhling  eel,  which  Dr.  Priestley  conjectures  to  be 
different  from  the  gymnotus ;  it  lives  in  marshy 
places,  from  whence  it  cannot  be  taken,  except  when 
it  is  intoxicated.  It  cannot  be  touched  with  the 
hand,  or  with  a  stick,  without  feeling  a  terrible  shock. 
If  trod  upon  with  shoes,  the  legs  and  thighs  are 
affected  in  a  similar  manner. 

Humboldt,  the  celebrated  traveller,  when  journey- 
ing across  the  Llanos,  in  South  America,  was  in- 
formed, that  in  the  neighbourhood  of  the  small  town 
of  Calaboza,  at  a  place  called  the  Cano  de  Bera,  the 
gymnoti  were  very  numerous  ;  and  being  desirous  to 
obtain  some  of  them  to  make  experiments  upon,  he 
was  conducted  to  a  small  piece  of  water,  shallow, 
stagnant,  and  muddy,  but  of  the  heat  of  79  degrees. 
Understanding  from  the  natives  that  the  only  way  in 


SUMMARY,  i&c.  491 
which  they  could  be  cauglit  was  by  driving  horses 
and  mules  into  the  water  to  disturb  them,  and  cause 
them  first  to  expend  their  electric  power,  he  ordered 
about  thirty  horses  and  mules  to  be  collected  and 
driven  into  the  water  ;  the  natives,  by  means  of  long 
bamboos  or  harpoons,  preventing  their  escape.  The 
gymnoti,  roused  from  their  slumbers  by  this  noise  and 
tumult,  mount  near  the  surface,  and  swimming  like 
so  many  livid  water  serpents,  briskly  pursue  the  in- 
truders, and,  gliding  under  their  bellies,  discharge 
through  them  the  most  violent  and  repeated  shocks. 
The  horses,  convulsed  and  terrified,  their  manes 
erect,  and  their  eyes  staring  with  pain,  make  in- 
effectual struggles  to  escape.  In  a  few  minutes  Iwo 
of  them  sunk  under  the  water  and  were  drowned: 
but  the  surviving  horses  gradually  recovered  from 
the  shocks,  and  became  more  composed  and  vigorous. 
In  a  quarter  of  an  hour  the  gymnoti  had  expended 
their  power,  and  were  then  in  such  a  state  of  languor 
and  exhaustion,  that  they  were  easily  taken. 

CONVERSATION  XVI. 

GENERAL  SUMMARY  OF  ELECTRICITY,  WITH 
EXPERIMENTS, 

r.  You  now  understand  what  electricity  is  ? 

C.  Yes;  it  is  a  fluid  which  seems  to  pervade  all 
substances,  and,  when  undisturbed,  it  remains  in  a 
state  of  equilibrium. 

J.  And  that  certain  portion  which  every  body  is 
supposed  to  contain  is  called  its  natural  share. 

r.  When  a  body  is  possessed  of  more,  or  retains 
less,  than  its  natural  share,  it  is  said  to  be  charged,  or 
electrified. 

C.  If  it  possess  more  than  its  natural  share,  it  is 
said  to  be  positively  electrified,  but  if  it  contain  less 
than  its  natural  share,  it  is  said  to  be  negatively 
electrified. 


492 


ELECTRICITY. 


T,  Does  it  not  sometimes  happen,  that  the  same 
substance  is  both  positively  and  negatively  electrified 
at  the  same  time? 

J.  Yes:  the  Ley  den  jar  is  a  striking  instance  of 
this,  in  which,  if  the  inside  contain  more  than  its 
natural  share,  the  outside  contains  less  than  its  natural 
quantity. 

T.  What  is  the  distinction  between  conductors  and 
non-conductors  of  electricity? 

C.  The  electric  fluid  passes  freely  through  the 
former,  but  the  latter  oppose  its  passage. 

r.  You  know  that  electricity  is  excited  in  the 
greatest  quantities  by  the  friction  of  conducting  and 
non-conducting  substances  against  each  other. 

Ex.  Rub  two  pieces  of  sealing-wax,  or  two  pieces 
of  glass,  together,  and  only  a  very  small  portion  of 
electricity  can  be  obtained  ;  therefore,  the  rubber  of 
a  machine  should  be  a  conducting  substance,  and 
not  insulated. 

Every  electrical  machine,  with  an  insulated 
rubber,  will  act  in  three  different  ways  ;  the  rubber 
will  product  negative  electricity  :  the  conductor  will 
give  out  positive  electricity  ;  and  it  will  communicate 
both  powers  at  once  to  a  person  or  substance  placed 
between  two  directors  connected  with  them. 

/.  How  does  the  rubber  produce  negative  elec- 
tricity ? 

T.  If  you  Stand  on  a  stool  with  glass  legs,  or  upon 
any  other  non-conducting  substance,  and  lay  hold 
of  the  rubber,  or  a  chain  that  communicates  with  it, 
the  working  the  machine  will  take  away  from  you  a 
quantity  of  your  natural  electricity;  therefore  you 
will  be  negatively  electrified. 

C.  Will  this  appear  by  the  nature  of  the  electric 
fluid,  if  I  hold  in  my  hand  a  steel  point,  as  a  needle  ? 

T.  If  you.  Standing  on  a  non-conducting  sub- 
stance, are  connected  with  the  rubber,  and  your 
brother,  in  a  similar  situation,  connected  with  the 
conductor,  hold  points  in  your  hands,  and  I,  while  I 


SUMMARY,  &c. 


493 


stand  on  the  ground,  first  present  a  brass  ball,  or 
other  substance,  to  the  needle  in  your  hand,  and 
then  to  that  in  his  hand,  the  appearance  of  the 
fluid  will  be  different  in  both  cases ;  to  the  needle  in 
your  hand  it  will  appear  like  a  star,  but  to  that  in 
your  brother's  it  will  be  rather  in  the  form  of  a 
brush. — What  will  happen  if  you  bring  two  bodies 
near  to  one  another  that  are  both  electrified 

J.  If  they  are  both  positively  or  both  negatively 
electrified,  they  will  repel  each  other,  but  if  one  is 
negative  and  the  other  positive,  they  will  attract  one 
another  till  they  touch,  and  the  equilibrium  is  again 
restored. 

T.  If  a  body,  containing  only  its  natural  share  of 
electricity,  be  brought  near  to  another  that  is  elec- 
trified, what  will  be  the  consequence  ? 

C.  A  quantity  of  electricity  will  force  itself 
through  the  air  in  the  form  of  a  spark. 

T,  When  two  bodies  approach  each  other,  one 


superabundant  electricity  rushes  violently  from  one 
to  the  other  to  restore  the  equilibrium.  What  will 
happen  if  your  body,  or  any  part  of  it,  form  part 
of  the  circuit  1 

J.  It  will  produce  an  electric  shock,  and  if,  instead 
of  one  person  alone,  many  join  hands,  and  form  a 
part  of  the  circuit,  they  will  all  receive  a  shock  at 
one  and  the  same  instant. 

r.  If  I  throw  a  larger  quantity  of  electricity  than 
its  natural  share  on  one  side  of  a  piece  of  glass,  what 
will  happen  to  the  other  side  ? 

C.  The  other  side  will  become  negatively  electri- 
fied ;  that  is,  it  will  have  as  much  less  than  its 
natural  share,  as  the  other  has  more  than  its  natural 
share. 

T.  Does  electricity,  communicated  to  glass,  spread 
over  the  whole  surface? 

J.  No,  glass  being  an  excellent  non-conductur,  the 
electric  fluid  will  be  confined  to  the  part  on  which  it 
is  thrown  ;  and  for  that  reason,  and  in  order  to  apply 


electrified  positively 


negatively,  the 


494  ELECTRICITY. 

it  to  the  whole  surface,  the  glass  is  covered  with  tin- 
foil, which  is  called  a  coating, 

T.  And  if  a  conducting  communication  be  made 
between  both  sides  of  the  glass,  what  takes  place 
then  ?  ^ 

C,  A  discharge  ;  and  this  happens  whether  the 
glass  be  flat,  or  in  any  other  form. 

T,  What  do  you  call  a  cylindrical  glass  vessel 
thus  coated  for  electrical  purposes  1 

J,  A  Leyden  jar;  and  when  the  insides,  and  also 
the  outsides,  of  several  of  these  jars  are  connected,  it 
is  called  an  electrical  battery.  ' 

r.  Electricity,  in  this  form,  is  capable  of  pro- 
ducmg  the  most  powerful  effects,  such  as  melting 
metals,  firing  spirits,  and  other  inflammable  sub- 
stances.—What  efl'ect  has  metallic  points  upon 
electricity?  ^ 

C.  They  discharge  it  silently,  and  hence  their 
great  utility  in  defending  buildings  from  the  dire 
efl'ects  of  lightning.— Pray  what  is  thunder  ? 

T.  As  lightning  appears  to  be  the  rapid  motion  of 
vast  masses  of  electric  matter,  so  thunder  is  the  noise 
produced  by  the  motion  of  lightning:  and  when 
electricity  passes  through  the  higher  parts  of  the 
atmosphere,  where  the  air  is  very  much  rarefied,  it 
constitutes  the  aurora  borealis. 

Ex.  If  two  sharp-pointed  wires 
be  bent,   with  the  four  ends  at 
right  angles,  but  pointing  differ- 
ent ways,  and  they  be  made  to 
turn  upon  a  wire  x  fixed  on  the 
conductor,  the  moment  it  is  elec- 
trified a  flame  will  be  seen  at  the        Fig.  21. 
points  a  bed;  the  wire  will  begin 
to  turn  round  in  the  direction  opposite  to  that  to 
which  the  points  are  turned,  and  the  motion  will 
become  very  rapid. 

If  the  figures  of  horses,  cut  in  paper,  be  fastened 
upon  these  wires,  the  horses  will  seem  to  pursue  one 
another,  and  this  is  called  the  electrical  horse-race. 


SUMMARY,  &c.  495 
Of  course,  upon  this  principle,  many  other  amusing 
and  very  beautiful  experiments  may  be  made :  and 
upon  this  principle  several  electrical  orreries  have  been 
contrived,  shewing  the  motions  of  the  earth  and  moon, 
and  the  earth  and  planets  round  the  sun. 

J.  Hovi^  do  you  account  for  this  1  .  c 

T  Fix  a  sharp-pointed  wire  into  the  end  ot  the 
large  conductor,  and  hold  your  hand  near  it  :---no 
sparks  will  ensue;  but  a  cold  blast  wi  come  from  the 
point,  which  will  turn  any  light  mills,  wheels,  i<iC. 
with  great  velocity. 


GALVANISM,  or  VOLTAISM. 


CONVERSATION  I. 
OF  galvanism;  its  origin;  experiments — of  the 

DECOMPOSITION  OF  WATER. 

TUTOR  CHARLES  JAMES.  ^ 

! 

Tutor,  It  has  been  observed,  as  long  as  I  can 
remember,  and  probably  before  I  was  born,  that 
porter,  when  taken  from  a  pewter  pot,  had  a  superior 
flavour  than  when  drunk  out  of  glass  or  china. 

Charles,  Yes,  I  have  often  heard  my  uncle  say  so ; 
but  what  is  the  reason  of  it  ? 

T.  Admitting  the  fact,  which  is  I  believe  generally 
allowed  by  those  who  are  much  accustomed  to  that 
beverage,  it  is  now^  explained  upon  the  principles  of 
Galvanism, 

James,  Is  Galvanism  another  branch  of  science? 
Is  there  a  Galvanic  fluid  as  well  as  an  electric  fluid  1 

T,  Of  the  existence  of  the  electric  fluid  you  now 
have  no  doubt ;  the  science  of  electricity  took  its 
name  from  electron,  the  Greek  work  for  amber,  be- 
cause amber  was  one  of  the  first  substances  observed 
to  produce,  by  rubbing,  the  effects  of  attraction  and 
repulsion.  Galvanism  derives  its  name  from  Dr. 
Galvani,  who  first  reported  to  the  philosophical  world 
the  experiments  on  which  the  science  is  founded. 

C.  Pray  how  was  he  led  to  make  the  experi- 
ments? 


EXPERIMENTS.  49T 
T.  Galvani,  a  professor  of  anatomy  at  Bologna, 
was  one  evening-  making  some  electrical  experiments, 
and  on  the  table  where  the  machine  stood  were  some 
frogs  skinned :  by  an  accident,  one  of  the  company 
touched  the  main  nerve  of  a  frog,  at  the  same  mo- 
ment that  he  took  a  considerable  spark  from  the  con- 
ductor of  the  electrical  machine,  and  the  muscles  of 
the  frog  were  thrown  into  strong  convulsions.  These, 
which  were  observed  by  Galvani's  wife,  led  the  pro- 
fessor to  a  number  of  experiments,  but  as  they  cannot 
be  repeated  without  much  cruelty  to  living  animals, 
I  shall  not  enter  into  a  detail  of  them. 

J.  Were  not  the  frogs  dead  which  first  led  to  the 
discovery  ? 

r.  Yes,  they  were  :  but  the  Professor  afterwards 
made  many  experiments  upon  living  ones,  whence  he 
found,  that  the  convulsions,  or,  as  they  are  usually 
called,  the  contractions  produced  on  the  frog,  may  be 
excited  without  the  aid  of  any  apparent  electricity, 
merely  by  making  a  communication  between  the 
nerves  and  the  muscles  with  substances  that  are  con- 
ductors of  electricity. 

C.  Which  are  the  best  conducting  substances? 

T.  All  the  metals  :  but  zinc  and  silver,  or  zinc 
and  copper,  produce  the  greatest  effect, 

C,  Are  these  experiments  peculiar  to  frogs  1 

T.  No ;  they  have  been  successfully  made  on 
almost  all  kinds  of  animals,  from  the  ox  downwards  to 
the  fly.  And  hence  it  was  at  first  concluded,  that 
there  was  an  electricity  peculiar  to  animals. 

/.  You  have  already  shewn  that  the  electric  fluid 
exists  in  our  bodies,  and  may  be  taken  from  them,  in- 
dependently of  that  which  causes  the  contractions. 

T,  1  will  shew  you  an  experiment  on  this  subject : 
—here  is  a  thin  piece  of  zinCy  which  is  a  sort  of  me- 
tallic substance,  but  not  what  is  denominated  a  per- 
fect metal :  lay  it  under  your  tongue,  and  lay  this 
half-crown  mpon  the  tongue  ;  do  you  taste  any  thing 
very  peculiar  in  the  metals] 


498 


GALVANISM. 


/.  No,  nothing  at  all. 

T.  Put  them  in  the  same  position  again,  and  now 
bring  the  edges  of  the  two  metals  into  contact,  while 
the  other  parts  touch  the  under  and  upper  surfaces  of 
the  tongue. 

J.  Now  they  excite  a  very  disagreeable  taste, 
something  like  copperas. 

T.  Instead  of  the  half-crown,  try  the  experiment 
with  a  guinea,  or  with  a  piece  of  charcoal. 

C.  I  perceive  the  same  kind  of  taste  which  James 
described.    How  do  you  explain  the  fact  ? 

T.  Some  philosophers  maintain,  that  the  principle 
of  Galvanism  and  electricity  is  the  same  .  and  that  t 
the  former  is  the  evolution  or  emission  of  the  electric 
fluid  from  condncting  bodies,  disengaged  by  a  chemi- 
cal process  ;  while  the  latter  is  the  same  thing  made 
apparent  to  the  senses  by  non-conducting  bodies. 

J.  All  metals  are  conducting  substances  ;  of  course 
the  zinc,  the  guinea,  and  the  half-crown,  are  conduc- 
tors. 

T.  Yes,  and  so  are  the  tongue  and  the  saliva ; 
and  it  is  probable  that  by  the  decomposition  of  some 
small  particles  of  the  saliva  the  sharp  taste  is  ex- 
cited. ;, 

C.  What  do  you  mean  by  the  decomposition  of  | 
the  saliva  7  | 

T.  We  have,  in  our  chemistry,  shewn,  that  water  \ 
is  capable  of  being  decomposed,  that  is,  separated  into  J 
two  gases,  called  hydrogen  and  oxygen. 

J  Is  saliva  capable  of  being  thus  separated  1 

T.  Certainly ;  because  a  great  part  of  it  may  be 
supposed  to  be  water ;  and  the  oxygen  combines 
with  the  metal,  while  the  hydrogen  escapes,  and  ex- 
cites the  taste  on  the  tongue.  j 

C.  The  disagreeable  taste  on  the  tongue  cannot  be  I 
disputed,  but  there  is  no  apparent  change  on  the  zinc  ■ 
or  the  half-crown,  which  there  ought  to  be  if  a  new  I 
substance,  as  the  oxygen,  has  entered  into  the  com-  I 
bination.  m 


GALVANIC  LIGHT. 


499 


r.  The  change  is,  perhaps,  too  small  to  be  per- 
ceived in  this  experiment :  but  in  others,  on  a  larger 
scale,  it  will  be  very  evident  to  the  sight,  by  the  oxU 
dation  of  the  metals. 

J.  Here  is  another  strange  word  :  I  do  not  know 
what  is  meant  by  oxidation. 

T,  The  iron  bars  fixed  before  the  window  were 
clean  and  almost  bright  when  placed  there  last  sum- 
mer. 

J.  But  not  bemg  pamted  they  are  become  quite 
rusty. 

r.  Now  in  chemical  language  the  iron  is  said  to 
be  oxidated,  instead  of  rusty  ;  and  the  earthy  sub- 
stance that  may  be  scraped  from  them,  used  to  be 
called  the  calx  of  iron  ;  but  it  is,  by  modern  che- 
mistry, denominated  the  oxide  of  iron. 

When  mercury  loses  its  fine  brightness  by  being- 
long  exposed  to  the  air,  the  dulness  is  occasioned 
by  oxidation,  that  is,  the  same  effect  is  produced  by 
the  air  on  the  mercury,  as  was  on  the  iron.  I  will 
give  you  another  instance.  I  will  melt  some  lead  in 
this  ladle  ;  you  see  a  scum  is  speedily  formed.  I 
take  it  away,  and  another  will  arise,  and  so  perpe- 
tually till  the  whole  lead  is  thus  transformed  into  an 
apparently  different  substance ;  this  is  called  the 
oxide  of  lead,  and  the  process  is  called  oxidation.  On 
the  same  principle  oxides  of  all  the  metals  are  ob- 
tained ;  but  the  pure  metals,  as  gold  and  silver,  are 
not  easily  oxidated. 


CONVERSATION  II. 

GALVANIC  LIGHT,   AND  SHOCKS. 

C.  We  had  a  taste  of  the  Galvanic  fluid  yesterday  ; 
is  there  no  way  of  seeing  it  ? 

T.  Put  this  piece  of  zinc  between  the  upper  lip 
and  the  gums,  as  high  as  you  can,  and  then  lay  a 


50O 


GALVANISM. 


Ijalf-crown,  or  guinea,  upon  the  tongue,  and  when  so 
situated  bring  the  metals  into  contact. 
C.  I  thought  I  saw  a  faint  flash  of  light. 
T,  I  dare  say  you  did  ;  it  was  for  that  purpose  I 
bade  you  make  the  experiment.  It  may  be  done  in 
another  way ;  by  putting  a  piece  of  silver  up  one  of 
the  nostrils,  and  the  zinc  on  the  upper  part  of  the 
tongue,  and  then  bringing  the  metals  into  contact, 
the  same  effect  will  be  produced. 

/ .  By  continuing  the  contact  of  the  two  metals, 
the  appearance  of  light  does  not  remain. 

T.  No,  it  is  visible  only  at  the  moment  of  making 
the  contact.  You  may,  if  you  make  the  experiment ' 
with  great  attention,  put  a  small  slip  of  tin-foil  over ' 
the  ball  of  one  eye,  and  hold  a  teaspoon  in  your 
mouth,  and  then,  upon  the  communication  between 
the  spoon  and  the  tin,  a  faint  light  will  be  visible. 
These  experiments  are  best  performed  in  the  dark. 

C.  Is  there  no  means  of  making  experiments  on  a 
larger  scale  ? 

7'.  Yes,  we  have  Galvanic  batteries,  *  as  well  as 
electrical  batteries.  Here  is  one  of  them. 
It  consists  of  a  number  of  pieces  of  silver,      il  [1  [I 
zinc,  and  flannel  cloth,  of  equal  sizes  ;  j-^-^L^^ 
and  they  are  thus  arranged ;  a  piece  of 
zinc,  a  piece  of  silver,  and  a  piece  of 
cloth  moistened  with  a  solution  of  salt 
in  water,  and  so  on  till  the  pile  is  com- 
pleted.    To  prevent  the  pieces  from 
falling,  they  are  supported  on  the  sides 
by  three  rods  of  glass  fixed  into  a  piece 
of  wood,  and  down  these  rods  slides 
another  piece  of  wood,  which  keeps  all 
the  pieces  in  close  contact.  o*  * 

*  These  should  rather  be  called  Voltaic  batteries,  as 
Volta,  a  celebrated  Italian,  was  the  inventor  of  them  ; 
and  he  was,  in  fact,  from  his  great  discoveries,  the  real 
founder  of  this  science. 


GALVANIC  BATTERY.  501 
J.  How  do  you  make  use  of  this  instrument  1 
T,  Touch  the  lower  piece  of  metal  witli  one  hand, 
and  the  upper  one  with  the  other. 
J.  I  felt  an  electric  shock. 

T,  And  you  may  take  as  many  as  you  please ;  for 
as  often  as  you  renew  the  contact,  so  often  will  you 
feel  the  shock. 

Here  is  a  different  apparatus.    In  these  three 


Fig.  2. 


glasses  (and  I  might  use  twenty  instead  of  three)  is 
a  solution  of  salt  and  water.  Into  each,  except  the 
two  outer  ones,  is  plunged  a  small  plate  of  zinc,  and 
another  of  silver.  These  plates  are  made  to  commu- 
nicate with  each  other,  by  means  of  a  thin  wire, 
fastened  so  that  the  silver  of  the  first  glass  is  con- 
nected with  the  zinc  of  the  second ;  the  silver  of  the 
second  with  the  zinc  of  the  third,  and  so  on  :  now 
if  you  dip  one  hand  into  the  first  glass,  and  the  other 
into  the  last,  the  shock  is  felt. 

C.  Will  any  kind  of  glasses  answer  for  this  experi- 
ment ? 

T.  Yes,  they  will ;  wine-glasses,  or  goblets,  or 
finger-glasses ;  and  so  will  china  cups. 
A  third  kind  of  battery,  which  is  the  most  powerful, 


Fig  3. 


50*2 


GALVANISM. 


and  the  one  that  is  most  generally  used,  is  this.  It 
consists  of  a  trough  of  baked  wood,  three  inches  deep, 
and  about  as  broad.  In  the  sides  of  this  trough  are 
grooves  opposite  to  each  other,  and  about  a  quarter 
of  an  inch  asunder.  Into  each  pair  of  these  grooves 
is  put  a  plate  of  zinc,  and  another  of  silver,  and  they 
are  to  be  cemented  in  such  a  manner  as  to  prevent 
any  communication  between  the  different  cells.  The 
cells  are  now  filled  with  a  solution  of  salt  and  water. 
The  battery  is  complete  ;  with  your  hands  make  a 
communication  between  the  two  end  cells. 
C.  I  felt  a  strong  shock. 

T.  Wet  your  hands,  and  join  your  left  with  James's 
right,  then  put  your  right  hand  into  one  end  cell,  and 
let  James  put  his  left  into  the  opposite  one. 

J.  We  both  felt  the  shock,  like  an  electric  shock, 
but  not  so  severe. 

T.  Several  persons  may  receive  the  shock  together, 
by  joining  hands,  if  their  hands  are  well  moistened 
with  water.  The  strength  of  the  shock  is  much  dimi- 
nished by  passing  through  so  long  a  circuit.  The 
shock  from  a  battery  consisting  of  fifty  or  sixty  pairs 
of  zinc  and  silver,  or  zinc  and  copper,  may  be  felt  as 
high  as  the  elbows.  And  if  five  or  six  such  batte- 
ries be  united  with  metal  cramps,  the  combined  force 
of  the  shock  would  be  such  that  few  would  willingly 
take  it  a  second  time. 

C.  What  are  the  wires  for  at  each  end  of  the 
trough? 

T.  With  these  a  variety  of  experiments  may  be 
made  upon  combustible  bodies.  I  will  shew  you 
one  with  gunpowder,  but  I  must  have  recourse  to 
four  troughs,  united  by  cramps,  or  to  one  much  larger 
than  this. 

Towards  the  ends  of  the  wires  are  two  pieces  of 
glass  tubes :  these  are  for  the  operator  to  hold  by, 
while  he  directs  the  wires.  Suppose  now  four  or 
more  troughs  united,  and  the  wire  to  be  at  the  two 
extremities,  I  put  some  gunpowder  on  a  piece  of  flat 


GALVANIC  CONDUCTOR.  503 


glass,  and  then  holding  the  wires  by  the  glass  tubes, 
I  bring  the  ends  of  them  to  the  gunpowder,  and 
just  before  they  touch,  the  gunpowder  will  be  in- 
flamed. 

Instead  of  gunpowder,  gold  and  silver  leaf  may  be 
burnt  in  this  way  :  ether,  spirits  of  wine,  and  other 
inflammable  substances,  are  easily  fired  by  the  Gal- 
vanic battery ;  it  will  consume  even  small  metallic 
wires. 

Copper  or  brass-leaf,  commonly  called  Dutch  gold, 
burns  with  a  beautiful  green  light,  silver  with  a  pale 
blue  light,  and  gold  with  a  yellowish  green  light. 

J.  Will  the  battery  continue  to  act  any  great 
length  of  time  1 

T.  The  action  of  all  these  kinds  of  batteries  is  the 
strongest,  when  they  are  first  filled  with  the  fluid  ; 
and  it  declines  in  proportion  as  the  metals  are  oxi- 
dated, or  the  fluid  loses  its  power.  Of  course,  after  a 
certain  time,  the  fluid  must  be  changed  and  the  metals 
cleaned,  either  with  sand,  or  by  immersing  them  a 
short  time  in  diluted  muriatic  acid.  The  best  fluid 
for  filling  the  cells  with,  is  water  mixed  with  one- 
tenth  of  nitrous  acid.  Care  must  always  be  taken  to 
wipe  quite  dry  the  edges  of  the  plates,  to  prevent  a 
communication  between  the  cells  :  and  it  will  be 
found,  that  the  energy  of  the  battery  is  in  proportion 
to  the  rapidity  with  which  the  zinc  is  oxidated. 


CONVERSATION  III. 

GALVANIC    CONDUCTORS  CIRCLES  TABLES  

EXPERIMENTS. 

T.  You  know  that  conductors  of  the  electric  fluid 
differ  from  each  other  in  their  conducting  power. 

C.  Yes,  the  metals  were  the  most  perfect  conduc- 
tors, then  charcoal,  afterwards  water  and  other  fluids. 


504 


GALVANISM. 


We  remember  this  from  our  second  Conversation  on 
Electricity. 

T,  In  Galvanism  w^e  call  the  former  dry  and 
fect  conductors ;  these  are  the  first  class  :  'the  latter, 
or  second  class,  imperfect  conductors  ;  and  in  render- 
mg  the  Galvanic  power  sensible,  the  combination 
must  consist  of  at  least  three  conductors  of  the 
diflPerent  classes. 

Do  you  mean  two  of  the  first  class,  and  one  of 
the  second  ? 

T.  When  two  of  these  bodies  are  of  the  first  class, 
and  one  is  of  the  second,  the  combination  is  said  to 
be  of  the  first  order. 

C.  The  large  battery  which  you  used  yesterday 
was  of  the  first  order  then,  because  there  were  two 
metals,  viz.  zinc  and  silver,  and  one  fluid. 

T,  This  is  called  a  simple  Galvanic  circle ;  the  two 
metals  touched  each  other  in  some  points,  and  at 
other  points  they  were  connected  by  the  fluid,  which 
was  of  the  different  class. 

/.  Will  you  give  us  an  example  of  the  second 
order  1 

T.  When  a  person  drinks  porter  from  a  pewter 
mug,  the  moisture  of  his  under  lip  is  one  conductor 
of  the  second  class,  the  porter  is  the  other,  and  the 
metal  is  the  third  body,  or  conductor  of  the  first 
class. 

The  discoloration  of  a  silver  spoon,  in  the  act  of 
eating  eggs,  is  a  Galvanic  operation.  A  spoon  merely 
immersed  in  the  egg  undergoes  no  discoloration  j  it  is 
the  act  of  eating  that  produces  the  change.  This  is 
a  Galvanic  combination  of  the  second  order,  the  fluid 
egg  and  the  saliva  are  substances  of  the  second  class 
of  conductors,  and  the  silver  of  the  first  class. 

C.  Which  are  the  most  powerful  Galvanic  circles  ? 

T.  They  are  those  of  the  first  order,  where  two 
solids  of  different  degrees  of  oxidability  are  combined 
with  a  fluid  capable  of  oxidating  at  least  one  of  the 
solids.    Thus  gold,  silver,  and  water,  do  not  form  an 


GALVANIC  CIRCLES.  605 
•  active  Galvanic  circle,  but  it  will  become  active  if  a 
little  nitric  acid,  or  any  fluid  decomposible  by  silver, 
be  mixed  with  the  water.  An  active  Galvanic  circle 
is  formed  of  zinc,  silver,  and  water,  because  the  zinc 
IS  oxidated  by  water.  But  a  little  nitric  acid,  added 
to  the  water,  renders  the  combination  still  more 
active,  as  the  acid  acts  upon  the  silver  and  the 
zinc. 

The  most  powerful  Galvanic  combinations  of  the 
second  order  are,  where  two  conduc-tors  of  the  second 
class  have  different  chemical  actions  on  the  conduc- 
tors of  the  first  class,  at  the  same  time  that  they  act 
upon  each  other.  Thus  copper,  silver,  or  lead,  with  a 
solution  of  an  alkaline  sulphuret*  and  diluted  nitrous 
acid,  form  a  very  active  Galvanic  circle.  Hence  the 
following  Tables 


*  If  equal  quantities  of  sulphur  and  allcali  be  melted 
m  a  covered  crucible,  the  mass  obtained  is  called  an  al- 
K  aline  sulplmret. 

z 


500 


GALVANISM. 


TABLES. 


I.  Table  of  Galvanic  circles  of  the  first  order,  composed 
of  two  perfect  conductors,  and  one  imperfect  con- 
ductor. 

Very  OxUahle         Less  Oxidahle  Oxidating 
Substances.  Substances.  Fluids. 

f  With  g-old,  charcoal,  \  Solutions  of  ni- 
Zinc  ....  -J     silver,  copper,  tin,  J    trie  acid  in 

'  water,  of  mu- 
riatic acid, 
and  sulphu- 
ric acid,  &c. 
Water  holding* 
in  solution 
oxyg-en,  at- 
mospheric air 
&c. 

Solutions  of  ni- 
trate of  silver, 
and  mercury, 
nitric  acid, 
acetous  acid. 
Nitric  acid. 

II.  Table  of  Galvanic  circles  of  the  second  order,  com- 
posed of  two  imperfect  conductors,  and  one  perfect 
conductor. 

Perfect  Imperfect  Imperfect 

Conductors.  Conductors.  Conductors. 

Solution  of  ni- 
trous acid, 
oxyg-enated 
muriatic  acid, 
&c.  capable  of 
acting-  on  all 
the  metals. 


'  ir 


on,  mercury 

Iron  ....  5  With  ^old,  chai'coal, 
^     silver,  copper,  tin 

Tin    ....  5  With  g:old,  silver, 
^  charcoal 

Lead  ....  With  gold,  silver 
Copper  .  .  .  With  gold,  silver 
Silver    .    .    .     With  gold 


Charcoal 
Copper 
Silver 
Lead  . 
Tin  . 
Iron  . 
Zinc  . 


\  Solutions  of  hydro-  i 

g  g-enated    alkaline  * 

W  sulphurets,  capa- 

\  ble  of   acting  on 

(  the  first  three  me- 

^  tals,  but  not  on  the  ( 

J  last  three. 


EXPERIMENTS. 


507 


I  will  now  shew  you  another  experiment,  which  is 
to  be  made  with  the  assistance  of  the  great 
battery,    a  b  exhibits  a  glass  tube  filled 
with  distilled  water,  and  having  a  cork  at  [L\ 
each  end.   a  and  b  are  two  pieces  of  brass 
wire,  which  are  brought  to  within  an  inch  V 
or  two  of  one  another  in  the  tube,  and  the  ^ 
other  ends  are  carried  to  the  battery,  viz. 
A  to  what  is  called  the  positive  end,  and  b 
to  the  negative  end. 

/.  You  have  then  positive  and  negative 
Galvanism  as  well  as  electricity  ? 

T.  Yes,  and  if  the  circuit  be  interrupted, 
the  process  will  not  go  on.    But  if  all 
things  be  as  I  have  just  described,  you    Fig.  4, 
will  see  a  constant  stream  of  bubbles  of 
gas  proceed  from  the  wire  b,  which  will  ascend  to  the 
upper  part  of  the  tube.    This  gas  is  found  to  be 
hydrogen,  or  inflammable  air. 

C.  How  is  that  ascertained  1 

T.  By  bringing  a  candle  close  to  the  opening  when 
I  take  out  the  cork  a,  the  gas  will  immediately  in- 
flam£.  The  bubbles  which  proceed  from  the  wire  a 
are  oxygen,  or  pure  air,  they  accumulate  and  stick 
about  the  sides  of  the  tube. 

J.  How  is  this  experiment  explained? 

T.  It  is  believed  that  the  water  is  decomposed,  or 
divided  into  hydrogen  and  oxygen  :  the  hydrogen  is 
separated  from  the  water  by  the  wire  connected  with 
the  negative  extremity,  while  the  oxygen  unites  with 
and  oxidates  the  wire  connected  with  the  positive  end 
of  the  battery. 

If  I  connect  the  positive  end  of  the  battery  with 
the  lower  wire,  and  the  negative  with  the  upper,  then 
the  hydrogen  proceeds  from  the  upper  wire,  and  the 
lower  wire  is  oxidated. 

If  wires  of  gold  or  platina  be  used,  which  are  not 
oxidable,  then  a  stream  of  gas  issues  from  each, 
which  may  be  collected,  and  will  be  found  to  be  a 
mixture  of  hydrogen  and  oxygen. 


508 


GALVANISM. 


C.  Are  tliere  no  means  of  collecting  these  fluids 
separately  ? 

T.  Yes  ;  instead  of  making  use  of  the  tube,  let 
the  extremities  of  the  wires,  which  proceed  from 
the  battery,  be  immersed  in  water,  at  the  distance  of 
an  inch  from  each  other ;  then  suspend  over  each 
a  glass  vessel,  inverted,  and  full  of  water,  and  the 


difTerent  kinds  of  gas  will  bo  found  in  the  two 
glasses. 

It  is  known  that  hydrogen  gas  reduces  the  oxides 
of  metals,  that  is,  restores  them  to  their  metallic  state. 
If,  therefore,  the  tube  (Fig.  4.)  be  filled  with  a  solu- 
tion of  acetite  of  lead  *  in  distilled  water,  and  a  com- 
munication be  made  with  the  battery,  no  gas  is  per- 
ceived to  issue  from  the  wire  which  proceeds  from 
the  negative  end  of  the  battery,  but  in  a  few  minutes 
beautiful  metallic  needles  may  be  seen  on  the  ex- 
tremity of  the  wire. 

J.  Is  this  the  lead  separated  from  the  fluid  1 

T.  It  is;  and  you  perceive  it  is  in  a  perfect 
metallic  state,  and  very  brilliant.  Let  the  operation 
proceed,  and  these  needles  will  assume  the  form  of 
fern,  or  some  other  vegetable  substance. 

The  spark  from  a  Galvanic  battery  acts  with  won- 
derful activity  upon  all  inflammable  bodies ;  and  ex- 
periments made  in  a  dark  room,  upon  gunpowder, 
charcoal,  metallic  wire,  and  metallic  leaves,  &c.  may- 
be made  very  amusing. 


Fig.  5. 


•  Acetite  of  lead  is  a  solution  of  lead  in  acetous  acid. 


EXPERIMENTS. 


509 


C.  Is  not  the  operation  of  the  battery  very 
powerful  ? 

T.  It  is  ;  it  would  appear  that  the  heat  produced 
by  the  Galvanic  battery  is  more  intense  than  can  be 
excited  by  any  other  process.  In  the  experiments 
detailed  by  Mr.  Children,  the  action  of  his  apparatus 
raised  to  a  red  heat,  visible  in  full  day-light,  the 
whole  of  a  wire  of  platina,  one  tenth  of  an  inch  in 
diameter,  and  five  feet  and  a  half  in  length.  It  also 
effected  the  fusion  of  a  variety  of  substances,  on 
which  the  heat  of  the  best  wind-furnaces  makes  no 
impression. 

/.  The  heat  must,  then,  have  been  extreme. 

T,  It  must:  indeed,  if  the  battery  retains  its  power 
there  appears  to  be  no  limit  to  the  continual  evolu- 
tion of  heat.  When  any  thin  metallic  leaves  are 
placed  in  the  electric  current  of  a  powerful  battery, 
they  take  fire,  and  by  continuing  the  action  they  may 
be  made  to  burn  with  great  brilliancy. 


CONVERSATION  IV. 

MISCELLANEOUS  EXPERIMENTS, 

T.  The  discoveries  of  Galvani  were  made  principally 
with  dead  frogs  j  from  his  experiments,  and  many 
others  that  have  been  made  since 'his  time,  it  appears 
that  the  nerves  of  animals  may  be  affected  by  smaller 
quantities  of  electricity  than  any  other  substances 
with  which  we  are  acquainted.  Hence  limbs  of  ani- ' 
mals,  properly  prepared,  have  been  much,  employed 
for  ascertaining  the  Galvanic  electricity. 

C.  What  is  the  method  of  preparation  ? 

T.  I  have  been  cautious  in  mentioning  experiments 
on  animals,  lest  they  should  lead  you  to  trifle  with 
their  feelings  ;  I  must,  however,  to  render  the  subject 
more  complete,  tell  you  what  has  been  done. 

The  muscles  of  a  frog  lately  dead,  and  skinned. 


6K) 


GALVANISM. 


may  be  brought  into  action  by  means  of  very  small 
quantities  of  common  electricity. 

If  the  leg  of  a  frog  recently  dead  be  prepared,  that 
is,  separated  from  the  rest  of  the  body,  having  a 
small  portion  of  the  spine  attached  to  it,  and  so 
situated  that  a  little  electricity  may  pass  through  it, 
the  leg  will  be  instantly  affected  with  a  kind  of  spas- 
modic contraction,  sometimes  so  strong  as  to  jump  a 
considerable  distance. 

It  is  now  known  that  similar  effects  may  be  pro- 
duced in  the  limb  thus  prepared,  by  only  making  a 
communication  between  the  nerves  and  the  muscles  by 
a  conducting  substance.  Thus,  in  an  animal  recently 
dead,  if  a  nerve  be  detached  from  the  surrounding 
parts,  and  the  coverings  be  removed  from  ovei  the 
muscles  which  depend  on  that  nerve,  and  if  a  piece 
of  metal,  as  a  wire,  touch  the  nerve  with  one  ex- 
tremity, and  the  muscle  with  the  other,  the  limb  will 
be  convulsed. 

C.  Is  it  necessary  that  the  communication  between 
the  nerve  and  the  muscle  should  be  made  with  a  con- 
ducting substance  ? 

T.  Yes,  it  is  :  for  if  sealing-wax,  glass,  &c.  be 
used  instead  of  metals,  no  motion  will  be  produced. 

If  part  of  the  nerve  of  a  prepared  limb  be  wrapped 
up  in  a  slip  of  tin-foil,  or  be  laid  on  a  piece  of  zinc, 
and  a  piece  of  silver  be  laid  with  one  end  upon  the 
muscle,  and  with  the  other  on  the  tin  or  zinc,  the 
motion  of  the  limb  will  be  very  violent. 

Here  are  two  wine-glasses,  almost  full  of  water  ; 
and  so  near  to  each  other  as  barely  not  to  touch  :  I 
put  the  prepared  limb  of  the  frog  into  one  glass,  and 
lay  the  nerve,  which  is  wrapped  up  in  tin-foil,  over 
the  edges  of  the  two  glasses,  so  that  the  tin  may 
touch  the  water  of  the  glass  in  which  the  limb  is  not. 
If  I  now  form  a  communication  between  the  water  in 
the  two  glasses,  by  means  of  silver,  as  a  pair  of  sugar 
tongs ;  or  put  the  fingers  of  one  hand  into  the  water 
of  the  glass  that  contains  the  leg,  and  hold  a  piece  of 


EXPERIMENTS.  511 
silver  in  the  other,  so  as  to  touch  the  coating  of  the 
nerves  with  it,  the  limb  will  be  immediately  excited, 
and  sometimes,  when  the  experiment  is  well  made,  the 
leg  will  even  jump  out  of  the  glass. 

/.  It  is  very  surprising  that  such  kind  of  motions 
should  be  produced  in  dead  animals. 

T,  They  may  be  excited  also  in  living  ones  :  if  a 
live  frog  be  placed  on  a  plate  of  zinc,  having  a  slip  of 
tm-foil  upon  its  back,  and  a  communication  be  made 
between  the  zinc  and  tin-foil,  by  a  piece  of  metal,  as 
silver,  the  same  kind  of  contractions  will  take  place. 

C.  Can  this  experiment  be  made  without  injury  to 
the  animal '? 

T,  Yes,  and  so  may  the  following :  I  take  a  live 
flounder,  and  dry  it  with  a  cloth,  and  then  put  it  in  a 
pewter  plate,  or  upon  a  large  piece  of  tin-foil,  and 
place  a  piece  of  silver  on  its  back ;  I  now  make  a 
communication  between  the  metals  with  any  con- 
ducting substance,  and  you  see  the  contractions,  and 
the  fish's  uneasiness.  The  fish  may  now  be  replaced 
in  water. 

I  place  this  leech  on  a  crown  piece,  and  then,  in 
its  endeavour  to  move  away,  let  it  touch  a  piece  of 
zinc  with  its  mouth,  and  you  will  see  it  instantly  re- 
coil, as  if  in  great  pain  :  the  same  thing  may  be  done 
with  a  worm. 

It  is  believed  that  all  animals,  whether  small  or 
great,  may  be  affected,  in  some  such  manner,  by 
Galvanism,  though  in  different  degrees. 

The  limbs  of  people,  while  undergoing  4;he  opera- 
tion of  amputation,  have  been  convulsed  by  the  ap- 
plication of  the  instruments,  an  effect  which  is  easily 
explained  by  Galvanism. 

By  the  knowledge  already  obtained  in  this  science, 
the  following  facts  are  readily  explained  : — 

Pure  mercury  retains  its  metallic  splendour  during 
a  long  time  ;  but  its  amalgam  with  any  other  metal  is 
soon  tarnished  or  oxidated. 

Ancient  inscriptions  engraved  upon  pure  lead  are 


512  GALVANISM. 

preserved  to  this  day,  whereas  some  medals  composed 

of  lead  and  tin,  of  no  great  antiquity,  are  very  much 

corroded. 

Works  of  metal,  whose  parts  are  soldered  together 
by  the  interposition  of  other  metals,  soon  oxidate 
about  the  parts  where  the  different  metals  are  joined. 
And  there  are  persons  who  profess  to  find  out  seams 
m  brass  and  copper  vessels  by  the  tongue,  which  the 
eye  cannot  discover,  and  they  can,  by  this  means,  dis- 
tmguish  the  base  mixtures  which  abound  in  gold  and 
silver  trinkets. 

When  the  copper  sheeting  of  ships  is  fastened  on 
by  means  of  iron  nails,  those  nails,  but  particularly 
the  copper,  are  very  quickly  corroded  about  the  placo 
v{  contact. 

A  piece  of  zinc  may  be  kept  in  water  a  long  time, 
without  scarcely  oxidating  at  all ;  but  the  oxidation 
takes  place  very  soon  if  a  piece  of  silver  touch  the 
zinc,  while  standing  in  the  water. 

If  a  cup  made  of  zinc  or  tin  be  filled  with  water, 
and  placed  upon  a  silver  waiter,  and  the  tip  of  the 
tongue  be  applied  to  the  water,  it  is  found  to  be  in- 
sipid ;  but  if  the  waiter  be  held  in  the  hand,  which  is 
well  moistened  with  water,  and  the  tongue  applied  as 
before,  an  acid  taste  will  be  perceived. 

C.  Is  that  owing  to  the  circuit  being  made  com- 
plete by  the  wet  hand  ? 

T.  It  is;  another  experiment  of  a  similar  kind  is  the 
following :  if  a  tin  bason  be  filled  v/ith  soap-suds,  lime- 
water,  or  a  strong  ley,  and  then  the  bason  be  held 
in  both  hands,  moistened  with  pure  water,  while  the 
tongue  is  applied  to  the  fluid  in  the  bason,  an  acid  taste 
will  be  sensibly  perceived,  though  the  liquor  is  alkaline. 
From  this  short  account  of  Galvanism  it  may  be 
inferred : — • 

(1.)  That  it  appears  to  be  only  another  mode  of 
exciting  electricity. 

(2.)  Galvanic  electricity  is  produced  by  the  che- 
mical action  of  bodies  upon  each  other. 


EXPERIMENTS.  513 

(3.)  The  oxidation  of  metals  appears  to  produce  it 
in  great  quantities. 

(4.)  Galvanic  electricity  can  be  made  to  set  in- 
flammable substances  on  fire,  to  oxidate  and  even  in- 
flame metals. 

(5.)  The  nerves  of  animals  appear  to  be  most 
easily  aiFected  by  it  of  any  known  substances. 

(6.)  Galvanic  electricity  is  conducted  by  the  same 
substances  as  common  electricity. 

(7.)  When  it  is  made  to  pass  through  an  animal, 
it  produces  a  sensation  resembling  the  electrical 
shock. 

(8.)  The  electricity  produced  by  the  torpedo  and 
electrical  ed  is  very  similar  to  Galvanism. 


INDEX  AND  GLOSSARY. 


A. 

ABSORB,  to  drink  in. 

Acceleration,  a  body  moving  faster  and  faster. 
Action  and  re-action,  equal  and  contrary,  42,  Curious 

instance  of,  43. 
Adhesion,  a  sticking  together. 

Air,  a  fluid,  the  pressure  of  which  is  very  great,  its 
nature  and  uses,  245.  Its  pressure,  experiments 
on,  252 — 261.  Its  weight,  how  proved,  261,  Its 
elasticity,  265—269.  Its  compression,  270—273. 
Necessary  to  sound,  279. 

Air-gun,  structure  of,  explained,  277. 

Air-pump  described,  247.  Its  structure  explained, 
248.    Experiments  on,  250,  259—274. 

Alcohol,  ardent  spirit :  equal  parts  of  alcohol  and 
water  make  spirits  of  wine. 

Alkaline,  a  saline  taste. 

Altitudes,  measured  by  the  barometer,  319. 

Anamorphoses,  distorted  images  of  bodies,  385. 

Ancients,  their  mode  of  describing  the  constellations, 
78. 

Angle,  what  it  is,  2.    How  explained,  ib.    Right  j 

obtuse  ;  acute,  3.    How  called,  4. 
Animals,  all  kinds  of,  affected  by  Galvanism,  511. 
Aperture,  a  small  hole. 


516 


GLOSSARY  AND  INDEX. 


Aphelkm,  the  greatest  distance  of  a  planet  from  the 
sun. 

Apogee^  the  sun's  or  moon  s  greatest  distance  from  the 
earth. 

Aquafortis,  of  what  composed,  5. 

Archimedes  proposed  to  move  the  earth,  46.  Some 

account  of,  213.  His  inventions,  213.  His  burning 

mirrors,  214. 
Arrow,  to  find  the  height  to  which  ascends,  25. 
Atmosphere,  height  of,  317.  Pressure  of  on  the  earth, 

323.    The  effect  of,  352,  365.    Light  refracted 

by,  365, 

Attraction,  the  tendency  which  some  parts  of  matter 

have  to  unite  with  others. 
Attraction,  capillary,  what  meant  by,  11.  Illu&trated, 

12. 

Attraction  and  Repulsion,  electrical,  434. 

  .  II  magnetic,  420,  &€. 

Aurora  Borealis,  vulgarly  called  the  Northern  lights. 

Its  use  in  the  Northern  parts  of  the  globe,  140. 

Imitated,  481.    A  curious  one  described,  ib. 

B. 

Balance,  bydrostatical,  described,  205. 

Balances,  false,  how  detected,  52. 

Ball,  why  easily  rolled,  28.   Scioptric,  its  effect,  361. 

Barometei'  explained,  263  and  313.    Its  construction, 

313.     Its  use,  ib.     Standard  altitude  of,  314. 

Variation  of,  315,     To  measure  altitudes  v.ith, 

319. 

Battery,  electrical,  described,  461.  Experiments  on, 
463. 

Beccaria,  his  observation  on  falling  staj'S,  480. 

Bellows,  bydrostatical,  183. 

Birds,  how  they  support  themselves  in  the  air,  37. 

Bissextile,  the  meaning  of  the  word,  122. 

Bodies,  heavenly,  why  move  in  a  curved  path,  42. 
Elastic  and  non-elastic,  illustrative  of  the  third  law 
of  motion.    Weight  of,  diminished  as  the  distance 


GLOSSARY  AND  INDEX. 


517 


from  the  centre  of  the  earth  is  increased,  15. 
Falling,  the  law  of  their  velocity,  24.  How  to  in- 
sulate, 442.  Sonorous,  elastic,  282.  Heavenly,  the 
latitude  of,  89.    Their  vis  inerti?e,  33. 

Body,  moving  one,  what  compels  it  to  stop,  35. 

Boyle,  Mr.  first  saw  the  electrical  light,  432. 

Bride's  (St.)  church,  damaged  by  lightning,  477. 

Bucket,  how  suspended  on  the  edge  of  a  table,  32. 

Bnffon,  M.  his  experiments,  372. 

Bullets,  leaden,  how  made  to  cohere,  8. 

Burning  Lenses,  355. 

C. 

Camera  ohscxira,  413. 
Cannon,  the  sound  of,  281. 

Capillary  attraction,  fluids  attracted  above  their  level, 

by  tubes  as  small  as  a  hair. 
Cardinal  points,  how  distinguished,  78. 
Cavallo,  Mr.  his  electrical  experiments,  476. 
Catoptrics,  the  science  of  reflected  light. 
Cements,  12. 

Centre  of  gravity y  the  point  of  a  body,  on  which, 
when  suspended,  it  will  rest,  27.  Between  the 
earth  and  sun,  107.  How  applicable  to  the  com- 
mon actions  of  life,  28. , 

Centrifugal  force,  is  the  tendency  which  a  body  has  to 
fly  off  in  a  straight  line. 

Centripetal  force,  is  the  tendency  which  a  body  has 
to  another  about  which  it  revolves. 

Chain-pump,  243. 

Circ/es,  Galvanic,  what,  504.   First  order,  ib.  Second 

order,  ib.  The  most  powerful,  505. 
Clepsydra,  principle  of,  explained,  192. 
Clocks  and  Dials,  why  not  agree  in  the  measure  of 

time,  89,  118. 
Cohesion,  attraction  of,  8.  How  defined,  9.  Instances, 

ib.    Its  force,  ib.    How  overcome,  ib.  Instances 

of,  11. 

Coining,  apparatus  for,  referred  to,  71, 


518 


GLOSSARY  AND  INDEX. 


Colours,  the  cause  of,  367. 

Comets,  in  what  respects  they  resemble  planets,  155. 

The  heat  of  one  calculated,  155. 
Compression,  the  act  of  squeezing  together. 
Concave  lenses,  362. 

—  mirrors,  356,  373,  376.     Experiments  with, 

384. 

Condensation,  the  act  of  bringing  the  parts  of  matter 
together. 

Conductors,  electrical,  what  meant  by,  436.  Table 
of,  437.  Galvanic,  503.  Perfect  and  imperfect, 
504. 

Cone,  double,  why  it  rolls  up  a  plane,  31. 
Conjunction,  planets  when  in,  126.    Moon  when  in, 
126. 

Contact,  touching. 

Converge,  drawing  towards  a  point. 

Convex  mirrors,  378,  385. 

Cookery,  some  operations  of,  how  accounted  for,  9. 
Crane,  the  principle  of  a,  58.     One  invented  by 

Mr.  White,  59.    Distiller's,  described,  230. 
Cupping,  the  operation  of,  explained,  269. 
Cups,  hemispherical,  experiments  on,  260. 
Cylinder,  how  made  to  roll  up  a  hill,  32. 

D. 

Dancers,  rope  or  wire,  how  they  balance  themselves, 
30. 

Dancing  figures,  electrical,  450. 

Day,  astronomical,  when  begins,  88.  The  difference 
between  the  solar  and  sidereal,  118. 

Day  and  Night,  how  explained,  103. 

Days  and  Nights,  why  of  different  lengths,  106.  To 
whom  always  equal,  110. 

Deception,  optical,  74.    In  feeling,  75. 

Deceptions,  on  the  public  by  short  weights,  how  de- 
tected, 52.  Occasioned  by  swift  motions,  101. 
Optical,  348—351,  383, 

Declination,  of  the  sun,  88.    Of  the  moon,  88. 


GLOSSARY  AND  INDEX. 


519 


Degrees,  how  subdivided,  86. 
Delaval,  Mr.  his  experiments,  369. 
Density,  compactness.    Constitutes  specific  gravity, 
202. 

Diagonal,  the  line  which  joins  the  opposite  corners  of 
a  square  or  other  right  lined  figure. 

Digester,  used  for  making  soups,  10. 

Dipping  of  the  needle  of  the  compass,  428. 

Direction,  line  of,  how  defined,  27.  Must  be  within 
the  base  of  a  body  that  stands  secure,  ib. 

Discharging  rod,  458. 

Distance,  measured  by  sound,  284. 

Diver's  Bell  described,  231.  How  used,  234.  Ac- 
cidents with,  234.  Smeaton's  improvements  on, 
235.  Walker's  improvements  on,  235.  Anecdote 
of,  236. 

Diverge,  to  spread  out. 

Drowning,  the  danger  of  to  inexperienced  persons, 
226. 

E. 

Earth,  centre  of,  why  bodies  tend  to  it,  15.  Why 
not  apparently  moved,  19.  Its  shape,  22.  Its 
diurnal  motion,  98.  103.  The  velocity  of  its 
motion,  102.  When  its  motion  is  quickest,  121. 
Its  annual  motion,  106 — 108.  Its  rotation,  the 
most  uniform  motion  in  nature,  118.  A  satellite 
to  the  moon,  127,  No  argument  against  its  mo- 
tion, because  not  apparent,  99.  Its  magnitude, 
102.  Its  globular  figure,  94.  How  proved,  95. 
Its  poles,  what,  97.  Its  axis,  97.  Nearer  the  sun 
in  winter,  112. 

Earthquakes,  484. 

Echo,  the  nature  of,  explained,  289.  Curious  ones 
noticed,  292.  Applied  to  the  measuring  of  dis- 
tances, 293. 

Eclipse,  an  occultation  of  the  sun  or  moon. 

Ec/ipses,  the  cause  of,  explained,  128.  Annular,  131. 
Total  of  the  sun,  very  rare,  131,    Account  of  one 


520 


GLOSSARY  AND  INDEX. 


seen  in  Portugal,  132.  Supposed  to  be  omens  of 
calamity,  132. 

Ecliptic,  the  earth's  annual  path  round  the  heavens. 
How  described,  82.    How  to  trace  the,  83. 

Effluvia,  fine  particles  that  fly  off  from  various  bodies. 

Eggs,  discoloration  of  silver  with  eating,  how  ex- 
plained, 504. 

Elasticity,  the  quality  in  some  bodies,  by  which  they 
recover  their  former  positions  after  being  bent. 
What  meant  by,  14. 

Electric,  what  meant  by,  431.  Light,  by  whom  first 
seen,  432. 

Electricity,  history  of,  431.  Attraction,  electrical, 
when  first  noticed,  432.  The  two  kinds,  446. 
Atmospheric,  475.  Medical,  485.  Animal,  487. 
Galvanic,  511. 

Electric  spark,  468. 

Electrical  discharger,  465. 

 experiments,  462,  468,  472,  492. 

 machine,  438. 

Electrometer,  453.   Lane^s,  460.    Quadrant,  the  use 

of,  462.    Another  kind,  474. 
Electrophorits,  474. 

Eolian  harp,  structure  of,  explained,  294. 
Ephemeris,  an  almanac.    White's,  explained,  85. 
Equator,  how  described,  83,  98. 
Equation  of  time,  117,  121. 
Equinoctial,  what  meant  by,  98. 

Evenings  at  Home"  referred  to,  a  work  of  great 

merit,  2. 

Eye,  the  parts  of  which  composed,  385. 

F. 

Fahrenheit's  thermometer,  321. 

Feathers,  electrified,  their  appearance,  448. 

Fire  Engines  described,  and  the  principle  of  them 

explained,  241. 
Fish,  how  they  swim,  246.    Air-vessel  of,  the  uses, 

247.    Electric,  453. 


GLOSSARY  AND  INDEX. 


521 


Flannel,  a  conductor  of  sound,  280. 
Flea,  circulation  of  the  blood  of  a,  7. 
Flood-gates,  why  made  very  thick,  192. 
Flowers,  colours  of,  369. 

Fluids  and  solids,  how  distinguished,  163,  Particles 
of  exceedingly  small,  164.  Incapable  of  com- 
pression, 165, 

Fluids  press  equally  in  all  directions,  169.  Incom- 
pressible, 169,  Air,  compression  of,  169.  Weight 
and  pressure  of,  experiments  on,  171.  Lateral 
pressure  of,  175.  Difference  between  the  weight 
and  pressure  of,  199.  Motion  of,  199.  Experiments 
on  the  light  and  heavy,  220.  Specific  gravity  of, 
differs  according  to  the  degrees  of  heat  and  cold, 
223. 

Focus,  354.    Imaginary  or  vertical  of  a  concave  lens, 

363.  Of  a  double  convex  lens,  363. 
Force,  opntrifugal,  what  meant  by,  39, 
Forcing'p2imp,  240. 

Fountains,  the  principle  of,  explained,  196, 
Fountain,  artificial,  272. 
Fountain  in  vacuo,  260. 

Franklin  (Dr.)  discovers  that  lightning  and  elec- 
tricity are  the  same,  475. 

Friction,  rubbing.  Must  be  allowed  for  in  me- 
chanics, 63. 

Frogs,  experiments  on,  509. 

Fulcrum,  the  prop  or  centre  on  which  a  lever  turns. 
What  meant  by,  48. 

G. 

Galvani,  (Dr.)  his  discoveries,  496.  Experiments  on 
frogs,  ib.  and  509. 

Galvanic  batteries,  how  formed,  500.'  Shock,  501. 

Galvanism,  what  it  is,  496.  From  what  it  derived  its 
name,  ib.  And  electricity  the  same,  499,  Made 
apparent  to  the  senses,  ib.    Positive  and  nega- 

'    tive,  507.  Summary  of,  512. 


622 


GLOSSARY  AND  INDEX. 


Garden  engines  described,  242. 

Gasy  a  kind  of  air.    Hydrogen,  how  procured,  507. 

How  collected,  508. 
Gauge,  a  measure. 

Geocentric  place  of  a  planet,  what  meant  by,  148. 

Longitude,  148. 
Globe,  the  greater  part  of  its  surface  water,  298.  A 

representation  of  the  earth,  97. 
Crlue,  for  what  used,  12. 

Gravity,  the  tendency  which  bodies  have  to  the 
centre  of  the  earth.  Centre  of,  what  meant  by, 
27.  How  found,  27.  Acts  upon  all  bodies,  15. 
The  law  of,  15  and  26.    Illustrated,  20  and  25. 

Gravitation,  attraction  of,  defined,  14.  Instances  of, 
ib.  By  this  force  bodies  tend  to  the  centre  of  the 
earth,  15. 

Gregory,  Pope,  rectifies  the  Julian  year,  123. 
Guinea,  specific  gravity  of,  205. 
Gunpowder,  how  fired,  502, 
Gymnotus,  described,  489.    How  caught,  490. 

H. 

Hammer,  philosophical,  252. 
Hampstead,  the  fine  prospect  from,  391. 
Harvest-moon  explained,  137.    Cause  of,  ib. 
Heat,  expands  all  bodies,  10.    The  cause  of  great, 

111.    Scale  of,  325. 
Height  of  any  place,  how  found,  24. 
Heliocentric  longitude,  148. 

Herschel,  the  planet,  when  discovered,  152.  Its 

magnitude,  distance,  &c.  93. 
Hiero's  crown,  cheat  respecting,  how  detected,  214. 
Hogshead,  how  burst,  185. 
Hook,  (Dr.)  his  microscope,  409. 
Hop-waggons,  dangerous  to  meet  on  an  inclining 

road,  30. 

Horizon,  the  boundary,  where  the  sky  seems  to  touch 
the  surface  of  the  earth  or  sea.    Sensible  and  ra- 


GLOSSARY  AND  INDEX. 


523 


tional,  103,  104.  To  which  we  refer  the  rising 
and  setting  of  the  sun,  104. 

Hydraulics,  hydrostatic  principles  applied  to  mills, 
engines,  pumps,  &c. 

Hydrometer,  an  instrument  to  measure  the  strength  of 
spirits.    Described,  220.    To  what  applied,  224. 

Hydrostatics,  the  origin  of  the  term,  162.  The 
objects  of,  ib. 

Hydrostatical  paradox,  explained,  178. 

Hydrostatical  bellows,  described  and  explained,  184. 
Press,  186  and  243.  Fluids,  pressure  of,  in  propor- 
tion to  the  perpendicular  heights,  187.  Balance, 
205. 

Hygrometer,  an  instrument  by  which  the  moisture  of 
the  air  is  measured.  Its  construction  and  use,  329. 
Different  kinds  of,  331. 

1.  J. 

Jack-a-lanthorn,  482. 
Immerse,  to  plunge  in. 
Impel,  to  drive  on. 

Incidence,  line  of,  289.    Angle  of,  343. 
Inclined  plane,  63. 

Incompressible,  not  capable  of  being  pressed  into  a 

sm.aller  compass. 
Inertia,  of  matter,  its  tendency  to  continue  in  the 

state  in  which  it  is. 
Ingenhouz  (Dr.)  referred  to,  13.    His  character,  ib. 
Interstices,  the  hollow  spaces  between  the  particles  of 

matter. 
Invisible  gii^l,  287. 
Iron,  oxide  of,  499. 

Jupiter,  the  planet,  its  magnitude  ;  distance  from  the 
sun  ;  the  velocity  of  its  motions,  149.  The  length 
of  its  days  and  nights,  ib.    Satellites,  ib. 

Jtdius  C(esar,  the  part  he  took  in  reforming  the  year, 
121. 


624 


GLOSSARY  AND  INDEX. 


L. 

Lateral,  sidewise. 

Latitude,  of  the  planets,  their  distance  from  the  eclip- 
tic, 36.    Parallels  of,  110. 

Lead,  eleven  times  heavier  than  water,  177,  201. 
Oxide  of,  499.    Acetite  of,  508. 

Leaf,  gold,  silver,  &c.  how  burnt,  503. 

Leaks f  in  banks,  how  secured,  193. 

Leap-year,  what  meant  by,  121.    Rule  for  knowing. 

Lenses f  different  kinds  described,  351. 

Levels,  construction  of,  169.    Use  of,  169. 

Lever,  a  bar,  crow,  &c.  For  what  used,  48.  Why 
called  a  mechanical  power,  48.  Of  the  first  kind, 
what  instruments  referred  to,  50.  How  to  estimate 
its  power,  52.  Of  the  second  kind,  what  instru- 
ments referred  to,  53.  Of  the  third  kind,  what 
instrum-ents  referred  to,  54. 

Levers,  how  many  kinds,  48.  Their  properties  illus- 
trated, ib. 

Ley  den  Phial,  454.  When  first  discovered,  456. 
Description  of,  457. 

Light,  its  great  velocity,  how  discovered,  150  and  339. 
Of  what  composed,  337.  Sun,  subject  to  no  appa- 
rent diminution,  338.  The  source  of  light  to  the 
planetary  worlds,  ib.  Moves  in  straight  lines,  340. 
Ray  of,  what  meant  by,  341.  Reflected  and  re- 
fracted, 342.  Its  great  advantages,  364.  A  com- 
pounded body,  366.  Galvanic,  how  perceived, 
500. 

Lightning,  conductors  for,  476.    Effects  of,  477. 
Lines,  right,  what  meant  by,  3,  note. 
Liquids  and  Fluids,  distinction  between,  163. 
London,  how  supplied  with  water,  197.  Bridge, 

water  works  at,  241, 
Long  days,  the  reason  of,  110. 
Longitude,  of  the  planets,  &c.  90.  Heliocentric, 

148. 


GLOSSARY  AND  INDEX. 
Lungs  glass,  275. 


525 


M. 

Machine,  electrical,  438.  The  most  powerful,  443. 
Magic  lanthorn,  415. 

Magnet,  described,  418.  Its  uses,  ib.  Directive  pro- 
perty,  ib.    Artificial,  420.    Properties  of,  ib. 

Magnets,  how  to  make,  423, 

Magnetic  attraction  and  repulsion,  420. 

Magnetism,  summary  of  facts  and  principles,  429. 

Marbles,  reason  why  they  roll  to  greater  or  less  dis- 
tances, 34. 

Mariner's  compass,  described,  427.  Variation  of,  427. 

Mars,  the  planet,  its  distance  from  the  sun;  its  velo^ 
city;  its  magnitude,  &c.  146. 

Matter,  every  substance  v/ith  which  we  are  ac- 
quainted. How  defined,  4.  Definition  illustrated, 
5.  Capable  of  infinite  division,  ib.  Remarkable 
instances  of  the  minute  division  of,  4—6. 

Mechanical  powers,  how  many,  and  what  they  are. 

Mechanics,  importance  of,  47.  Power  gained  bv 
them,  ib.  &  j 

Mercury,  the  planet,  its  situation,  140;  and  Venus, 
why  called  inferior  planets,  140.  Rarely  seen, 
141.  Its  distance  from  the  sun:  its  velocity;  its 
size,  &c.  ib.  ^ 

Metals,  some  more  sonorous  than  others,  282. 

Microscope,  its  principle  explained,  407.  Single, 
407.  How  made,  409.  Compound,  409.  Solar, 
412.  ^ 

Mirrors,  the  different  kinds,  370. 

Momenaim,  the  moving  force  of  a  body.  What 
by,  17.    Illustrated,  17,  44. 

Money,  counterfeit,  wrong  to  pass  it,  217. 

Month,  what  meant  by,  124.  Difference  between 
the  periodical  and  synodical,  124. 

Moon,  to  what  laws  subject,  21.    Its  declination,  88. 


526 


GLOSSARY  AND  INDEX. 


Its  southing,  88.  Its  distance  from  the  earth,  104. 
Probably  inhabited,  127.    Eclipses  of,  128. 

Moon  and  earth,  motions  of,  explained,  125.  Shines 
with  borrowed  light,  125.  The  length  of  her  dia- 
meter, 125.  Phases  of,  explained,  125.  Her 
rotation  described,  127.  Length  of  her  day,  ib. 
Length  of  her  year,  ib. 

Motion,  centre  of,  what  it  is,  45.  Laws  of,  33.  The 
first  illustrated,  ib.  The  second  illustrated,  36. 
The  third  illustrated,  ib.  Must  be  committed  to 
memory,  38. 

Moiious,  circular,  exist  in  nature,  38. 

Multiply ing-glass,  416. 

Muschenbroeck,  M.  describes  the  electric  shock,  456. 
Musical  instruments  depend  on  the  air  for  action,  294. 

N. 

Nadir,  the  point  under  our  feet. 

Nautical  Almanac,  its  use,  85. 

Needle,  of  the  mariner's  compass.    Dipping,  428. 

Nerves  and  muscles,  how  conductors  of  the  Galvanic 

fluid,  509. 
New  Style,  when  adopted,  123. 

Newton,  Sir  Isaac,  his  experiments  in  electricity,  432. 
Ninkler,  M.  his  description  of  the  electrical  shock, 
456. 

Nodes,  the  points  in  which  two  orbits  intersect  each 

other,  129. 
Non-conductors,  436. 
Non-elastic  Bodies,  42. 

O. 

Objects,  by  what  means  visible,  346.    The  image  of, 

how  painted  on  the  eye,  388. 
Oblate,  of  the  shape  of  an  orange. 
Opaque,  dark. 

Opposition,  when  the  moon  is  in,  126. 


OLOSSAHY  AND  INDEX. 


527 


Optical  delusions,  383. 

Orbit,  the  path  of  a  planet  round  the  sun,  or  of  a 

moon  round  its  primary.    The  earth's  orbit,  112. 
Orreries,  495. 

Oxidation,  what  meant  by  the  term,  499. 
Oxide,  the  calx  of  a  metal.    What  meant  by  the 
term,  499. 

P. 

Paley,  Dr.  his  Natural  Theoloofy  referred  to,  391. 
Papin's  Digester  described,  10.  311.     One  burst, 
312. 

Parker,  Mr.  his  large  burning-glass,  366. 
Pendulum,  71. 

Percussion,  a  stroke.    What  meant  by,  36. 

Phantasmagoria,  415. 

Phenomenon,  an  appearance  in  nature. 

Phial,  Leyden,  where  discovered,  456. 

Philosophy,  what  it  is,  1.  Natural  and  experimental, 
the  introduction  to,  not  difficult,  1.  Natural,  the 
uses  to  which  it  is  applied,  36. 

Philosophical  Transactions  referred  to,  480. 

Pisa,  tower  of,  leans  out  of  the  perpendicular,  29. 

Plane,  inclined,  explained,  63.  Examples  respect- 
ing the,  64.    What  instruments  referable  to,  65. 

Planets,  their  number  and  names,  92.  Characters  of, 
94.  Latitude  of,  89.  The  order  of  their  motions, 
93.  How  to  find  their  distances,  145.  Synopsis 
of,  154. 

Pneumatics,  what,  treated  of  under,  245. 
Points,  cardinal,  78. 
Pole-star,  its  use,  79. 

Poles,  apparently  stationary,  105.    At  the,  only  one 

day  and  one  night  in  the  year,  116. 
Press,  hydrostatical,  186. 

Priestley,  Dr.  his  history  of  electricity  referred  to, 
433. 

Price,  Dr.  referred  to,  22. 
Prism,  the  effect  of,  366,  396. 


528 


GLOSSARY  AND  INDEX. 


Pu(]dling,  what  meant  by  the  term,  193. 
Pulley,  how  explained,  60.    The  single  gives  no  ad- 
vantage, ib.    The  moveable,  ib. 
Parnp,  principle  of,  237. 
Pump  (^chain),  243. 

Pyrometer,  its  construction  and  use,  327. 

Q. 

Quicksilver f  pressure  of  a  column  of,  253. 

R, 

Radi ant-point Sf  from  whence  rays  of  light  flow  in  all 
directions.  ' 

Rainbow,  the  cause  explained,  396.    Artificial,  399. 

Curious  ones  described,  399. 
Rain-gauge,  its  construction,  332.    How  it  is  used, 

334. 

Rain,  an  electrical  phenomenon,  484. 

Rays,  pencil  of,  what  meant  by,  351.  Parallel,  defi- 
nition of,  351. 

Rejiection,  rebounding  back.  Its  power  in  apparently 
multiplying  objects,  77.  Line  of,  explained,  289. 
Of  light,  342. 

Refraction,  inclining  or  bending  out  of  its  course. 
Its  power  in  apparently  multiplying  objects,  77. 
Of  light,  344.  Optical  deceptions  arising  from, 
350. 

Repulsion  J  driving  away.  What  meant  by,  33.  In- 
stances of,  ib. 

Residuum,  electrical,  what  meant  by,  459. 

Retrograde  motion,  by  which  the  heavenly  bodies  ap- 
pear to  go  backwards. 

Reverberate,  to  beat  back. 

River,  New,  how  it  supplies  London  with  water,  197. 

Reservoirs  belonging  to,  197. 
Rivers,  banks  of,  must  be  very  thick,  193. 
Pope-pump,  242. 

Round-ahouts,  the  principle  of,  45. 


GLOSSARY  AND  INDEX. 


529 


Saliva,  decomposed  by  Galvanism,  498. 
Salt,  whatever  has  a  sharp  taste,  and  is  soluble  in 
water. 

Salt  water,  heavier  than  fresh,  consequence  of,  to  a 

loaded  vessel,  224. 
Satellites,  moons. 

Saturn,  the  planet,  how  known,  150.  Its  magnitude, 
distance  from  the  sun,  velocity  of  its  motions,  ib. 
Its  satellites  and  rings,  151 .  The  length  of  its  day 
and  night,  152. 

Savery,  (Capt.)  supposed  inventor  of  the  steam- 
engine,  303. 

Scioptric  ball,  361. 

Screw,  an  inclined  plane  wrapped  round  a  cylinder. 
Its  principle  explained,  67.  Of  what  composed, 
lb.  Examples  of,  68.  Used  by  paper-makers,  70. 
Its  power  estimated,  ib. 

Seasoji,  the  hottest,  113. 

Seasons,  variety  of,  on  what  depends,  108  and  111. 

Different,  how  accounted  for,  108—116.  How 

produced,  113. 
Shadow,  of  the  earth,  its  form,  130. 
Ship,  damaged  by  lightning,  480. 
Sight  and  smelling,  compared,  341. 
Silurus  electricus  described,  490. 
Silver,  experiment  with,  499. 
Slaves,  how  they  get  at  their  masters'  rum,  221. 
Smoke,  the  reason  of  its  ascent,  275. 
Smoke-Jack,  its  principle,  297. 
Solar  system,  described,  90. 
Solder,  for  what  used,  12. 

Soimd,  conductors  of,  289.  How  far  may  be  beard, 
282.  How  fast  it  travels,  284.  Velocity  of, 
applied  to  practical  purposes,  285. 

Southing,  moon's,  88. 

Spark,  electrical,  its  nature,  468.  Galvanic,  its 
power,  508. 

2  A 


530  GLOSSARY  AND  INDEX. 


Speaking  trumpet,  285. 
 images,  287. 

Specific  gravity,  what  meant  by,  173.    Of  bodies 

explained  and  illustrated,  199 — 220.   How  to  find, 

200—205.    Table  of,  219. 
Spectacles,  their  construction,   uses,   and  different 

kinds,  392. 
Spirit,  rectified,  what  meant  by,  223. 
Springs,  intermitting,  explained,  230. 
•St.  Paul's,  whispering  gallery  of,  principle  explained, 

293. 

Stars,  how  to  find  the  names  of,  80.  Why  marked 
on  the  globe  with  Greek  characters,  81.  Fixed, 
their  apparent  motion,  105.  Why  not  seen  in  the 
day,  106.  Fixed,  their  number,  74.  May  be 
distinguished,  78.  Fixed,  their  immense  distance, 
1 1 8.  Fixed,  description  of,  157.  Their  uses,  160. 
Falling,  what  they  are,  479. 

Steam-engine,  its  use,  302.  When  invented,  ib.  Its 
structure,  305.  The  application,  309.  That  of 
Messrs.  Whitbread  described,  310.  Its  power 
calculated,  ib.    Accidents  occasioned  by,  ib. 

Steel-yard,  a  sort  of  lever,  50.  Its  advantages  over  a 
pair  of  scales,  ib. 

Storms,  by  what  occasioned,  484. 

Style,  New  and  Old,  123. 

Suction,  no  such  principle  in  nature,  256. 

Sulphuret,  alkaline,  what,  505. 

Summers,  two  in  a  year,  in  some  places,  115. 

Sun  and  clocks,  seldom  together,  89. 

Sun,  declination  of,  88.  Longitude  of,  90.  Has  not 
latitude,  ib.  Its  magnitude,  91.  Why  it  appears 
so  small,  ib.  Its  distance  from  the  earth,  ib. 
Annual  motion  of,  how  observed,  82.  Reasons 
for,  ib.  Nearer  to  the  earth  in  winter  than  in  sum- 
mer, 112.  A  description  of,  157.  Eclipses  of,  131. 

Swimming,  theory  of,  225.  How  to  be  attained,  ib. 
Less  natural  to  man  than  to  other  land  animals,  225. 

Syphon,  the  structure  of,  explained,  227.  Its  princi- 
ple, 227 


GLOSSARY  AND  INDEX.  53I 

Syringe,  its  structure  explained,  254.  Condensing 
one  described,  272. 

T. 

Tables,  Galvanic,  506. 

Tangent,  a  straight  line  touching  the  circumference  of 

a  circle  in  one  point. 
Tangible,  capable  of  being  felt  or  handled. 
Tantalus's  cup,  229. 

Taste,  a  disagreeable  one,  excited  by  the  union  of 
metals  placed  on  and  under  the  tongue,  498.  How 
accounted  for,  ib. 

Telescope,  refracting,  explained,  400.  Night  404 
Reflecting,  explained,  404.    Dr.  Herschel's',  406! 

ierm,  technical,  derived  from  the  Greek  language, 

Thermometer,  its  construction  and  uses,  320  It« 
scale,  322.  Wedgewood's,  325.  Reaumur's  scale 
compared  with  Fahrenheit's,  327.    Heat,  scale  of 

Thunder,  how  produced,  494. 

Tides,  the  causes  of,  explained,  132.  Two  every 
twenty.five  hours,  135.  Different  in  difieren* 
places,  lb.    When  the  highest  happen,  136. 

Time,  equal  and  apparent,  how  distinguished,  117 
On  what  the  difference  depends,  117.  Equa- 
tion of,  89.    Division  of,  124. 

Tinw  and  space,  clear  ideas  of,  necessary  to  be  formed. 

Torpedo  described,  488. 

Torricellian  experiment,  254,  314. 

Transferrer,  an  instrument  used  in  pneumatics,  259 

J^ransH  of  Venus,  her  passages  over  the  sun's  face,  3. 

iremblijig-eel  noticed,  490. 

Triangle,  what  meant  by,  4.    Any  two  sides  of 

greater  than  the  third,  41. 
Tropics,  circles  parallel  to  the  equator. 
trumpet,  speaking,  described,  285.  When  first  used 

2o7. 


532 


GLOSSARY  AND  INDEX. 


Trumpets,  for  deaf  persons,  287, 
Tube,  a  pipe. 

Twilighty  the  degree  of  light  experienced  between  sun 
setting  or  rising  and  dark  night. 

U.  and  V. 

Undulation,  swinging  or  vibrating. 

Vacuum,  a  place  void  of  air. 

Valve,  a  sort  of  trap  door. 

Valves,  what  meant  by,  237. 

Vegetables,  how  blanched,  368. 

Velocity,  a  term  applied  to  motion.  Accelerating, 
what  meant  by,  23. 

Venus,  the  planet,  its  distance  from  the  sun  ;  the  ve- 
locity of  its  motion  ;  its  magnitude,  142.  Why  an 
evening  and  why  a  morning  star,  142.  Transit  of, 
what  meant  by,  142. 

Vernier,  its  construction  and  use,  315. 

Vertex,  the  top  of  any  thing. 

Vibratio7L,  the  swinging  motion  of  a  pendulum. 

Vis  Inerti(B,  39. 

Vision,  the  manner  of,  388. 

Volatile,  any  light  substance  that  easily  evaporates. 
Volcanoes,  in  the  moon,  128. 
Voltaic  Batteries. — See  Galvanic  Batteries. 
Voltaism. — See  Galvanism. 

W. 

Wall,  leaning  one,  at  Bridgenorth,  29. 

Water,  pure  rain,  the  standard  to  compare  other  bodies 

with,  201.    Weighs  the  same  every  where,  201. 

Always  deeper  than  it  appears  to  be,  226  and  349. 

How  raised  from  deep  wells,  242.    Formed  of  two 

gases,  498.    Decomposed,  507. 
Water-clocks,  192. 
Water 'press,  243. 

Water-spouts,  their  cause,  482.    How  dispersed,  483. 
Weather,  rules  for  judging  of,  334. 
Wedge,  a  triangular  piece  of  wood  or  metal,  to  cleave 
stone,  &c.  Its  principle  explained,  65.  Its  ad  van- 


GLOSSARY  AND  INDEX.  533 

tages  in  cleaving  wood,  ib.  What  instruments  re- 
ferred to,  ib. 

Wedgewood's  thermometer,  325. 

Weight  of  bodies  in  vacuo,  277. 

Well,  how  to  find  the  depth  of  one,  24. 

Wheel  and  axis  described,  57.  For  what  purposes 
usedjib.  Its  power  estimated,  58.  How  increased, 
lb.    Explained  on  the  principle  of  the  lever,  59. 

Whirlwinds,  483. 

Whispering-gallery,  293, 

White,  Mr.  James,  his  invention  of  a  crane,  59.  His 
patent  pulley,  62. 

Wind,  what  it  is,  296.  The  cause  of,  296.  Experi- 
ment on,  296  and  299.  Definition  of,  297.  Its 
direction  denominated,  ib.  The  cause  of  its  varia- 
bleness in  England,  300.  How  to  find  its  velocity. 
300.    Table  of,  302.  ^ 

Wind.gun,  278. 

Winds,  how  many  kinds,  and  why  so  named,  297. 
Winter,  why  colder  than  the  summer,  113. 
Wood  burned  to  a  coal  in  water,  356. 

Y. 

Year,  its  length,  how  measured,  121.  Gregorian 
what  meant  by,  121.  The  beginning  of,  changed 
from  the  25th  of  March  to  the  1st  of  January,  ill. 

Z. 

Zenith,  that  point  of  the  heavens  over  one's  head. 
Zino,  experiment  with,  497. 

Zodiac,  a  belt  in  the  heavens  sixteen  degrees  broad, 
through  which  the  ecliptic  runs.  Signs  of,  86. 
Dr.  Watts's  lines  on,  87. 


THE  END. 


4^  A,.  ^ 


LONDON  : 

PRINTED  BY  A.  SWEETING,  15,  BARTLETT's  BUILDINGS. 


UNIVERSITY  OF  ILLINOIS-URBANA 


3  0112  062107153 


